feat: threshold balloons + dark/light auto-follow

- Threshold balloons fire the cycle utilization crosses 80% or 95%
  on either provider, with the title showing "{Provider} · {N}%" and
  body translated per shipped locale. Reuses the existing
  BALLOON_COOLDOWN so notifications stay calm.
- Dark/light auto-follow: bubble's WM_SETTINGCHANGE handler now calls
  app::recheck_theme(), which re-reads HKCU\…\Personalize, updates
  state.is_dark if changed, and triggers a UI repaint + tray refresh.
  Windows posts WM_SETTINGCHANGE to every top-level window when the
  user toggles light/dark in Settings, so the bubble repaints in
  near-real-time.
- Adds 2 new i18n keys (threshold_80_body, threshold_95_body) across
  all eight shipped locales.

Bumps version to 0.1.5.

src/app.rs

@@ -468,39 +468,57 @@ fn apply_results(
     results: Vec<(ProviderId, Result<UsageWindows, usage::Error>)>,
 ) -> Vec<ProviderId> {
     let mut auth_failures = Vec::new();
-    let mut s = lock_state();
-    let Some(s) = s.as_mut() else {
-        return auth_failures;
-    };
-    if results.is_empty() {
-        return auth_failures;
-    }
-    let strings = s.i18n.strings().clone();
-    let mut any_ok = false;
-    for (id, outcome) in results {
-        match outcome {
-            Ok(windows) => {
-                let entry = s.snapshots.entry(id).or_default();
-                entry.windows = windows;
-                entry.primary_text = i18n::format_window(&windows.primary, &strings);
-                entry.secondary_text = i18n::format_window(&windows.secondary, &strings);
-                any_ok = true;
-            }
-            Err(usage::Error::AuthRequired | usage::Error::TokenExpired) => {
-                auth_failures.push(id);
-                let entry = s.snapshots.entry(id).or_default();
-                entry.primary_text = "!".into();
-                entry.secondary_text = "!".into();
-            }
-            Err(e) => {
-                log::warn!("provider {id:?} poll failed: {e}");
-                let entry = s.snapshots.entry(id).or_default();
-                entry.primary_text = "…".into();
-                entry.secondary_text = "…".into();
+    let mut crossings: Vec<(ProviderId, u8)> = Vec::new();
+    {
+        let mut guard = lock_state();
+        let Some(state) = guard.as_mut() else {
+            return auth_failures;
+        };
+        if results.is_empty() {
+            return auth_failures;
+        }
+        let strings = state.i18n.strings().clone();
+        let mut any_ok = false;
+        for (id, outcome) in results {
+            match outcome {
+                Ok(windows) => {
+                    let entry = state.snapshots.entry(id).or_default();
+                    let old_pct = entry.windows.primary.utilization;
+                    let new_pct = windows.primary.utilization;
+                    // Fire a balloon only the cycle a provider CROSSES a
+                    // threshold so the user is nudged once, not on every
+                    // subsequent poll while parked above it.
+                    for threshold in [80u8, 95u8] {
+                        if old_pct < threshold as f64 && new_pct >= threshold as f64 {
+                            crossings.push((id, threshold));
+                        }
+                    }
+                    entry.windows = windows;
+                    entry.primary_text = i18n::format_window(&windows.primary, &strings);
+                    entry.secondary_text = i18n::format_window(&windows.secondary, &strings);
+                    any_ok = true;
+                }
+                Err(usage::Error::AuthRequired | usage::Error::TokenExpired) => {
+                    auth_failures.push(id);
+                    let entry = state.snapshots.entry(id).or_default();
+                    entry.primary_text = "!".into();
+                    entry.secondary_text = "!".into();
+                }
+                Err(e) => {
+                    log::warn!("provider {id:?} poll failed: {e}");
+                    let entry = state.snapshots.entry(id).or_default();
+                    entry.primary_text = "…".into();
+                    entry.secondary_text = "…".into();
+                }
             }
         }
+        state.last_poll_ok = any_ok;
+    }
+    // Lock released. Fire any threshold balloons outside the critical
+    // section so tray::notify and i18n cloning don't block the UI thread.
+    for (id, threshold) in crossings {
+        show_threshold_balloon(id, threshold);
     }
-    s.last_poll_ok = any_ok;
     auth_failures
 }
 
@@ -752,6 +770,60 @@ fn handle_tray_action(action: TrayAction) {
     }
 }
 
+/// Re-read the system theme and, if it changed, push the new value into
+/// the UI. Called from each bubble's WM_SETTINGCHANGE handler — Windows
+/// posts that to every top-level window when the user toggles light/dark
+/// in Settings, so this naturally fires once per change.
+pub fn recheck_theme() {
+    let now_dark = os::theme::is_dark();
+    let changed = {
+        let mut s = lock_state();
+        let Some(s) = s.as_mut() else {
+            return;
+        };
+        if s.is_dark == now_dark {
+            false
+        } else {
+            s.is_dark = now_dark;
+            true
+        }
+    };
+    if changed {
+        propagate_to_ui();
+        refresh_tray_icons();
+    }
+}
+
+fn show_threshold_balloon(provider: ProviderId, threshold: u8) {
+    let payload = {
+        let mut s = lock_state();
+        let Some(s) = s.as_mut() else {
+            return;
+        };
+        // Reuse the same cooldown as the token-expired balloon: any one
+        // balloon at a time keeps notifications calm.
+        if let Some(last) = s.last_balloon_at {
+            if last.elapsed() < BALLOON_COOLDOWN {
+                return;
+            }
+        }
+        s.last_balloon_at = Some(Instant::now());
+        let strings = s.i18n.strings();
+        let provider_label = match provider {
+            ProviderId::Claude => strings.claude_label.clone(),
+            ProviderId::ChatGpt => strings.chatgpt_label.clone(),
+        };
+        let title = format!("{provider_label} · {threshold}%");
+        let body = if threshold >= 95 {
+            strings.threshold_95_body.clone()
+        } else {
+            strings.threshold_80_body.clone()
+        };
+        (s.msg_hwnd, provider, title, body)
+    };
+    tray::notify(payload.0.to_hwnd(), payload.1, &payload.2, &payload.3);
+}
+
 fn show_token_expired_balloon(failed: ProviderId) {
     let payload = {
         let mut s = lock_state();

src/bubble.rs

@@ -478,10 +478,13 @@ unsafe extern "system" fn wnd_proc(
             LRESULT(0)
         }
         WM_SETTINGCHANGE => {
-            // Taskbar move / auto-hide toggle / DPI change all post this.
-            // Just re-clamp into the new work area so the bubble can't end up
-            // hidden behind the new taskbar position.
+            // Taskbar move / auto-hide toggle / DPI change / theme toggle
+            // all post this. Re-clamp into the new work area (bubble must
+            // not end up hidden behind the new taskbar position) and ask
+            // the app to re-read the light/dark setting — Windows fires
+            // this message when the user flips the OS theme in Settings.
             clamp_into_work_area(hwnd);
+            crate::app::recheck_theme();
             LRESULT(0)
         }
         WM_DESTROY => {

src/i18n/locales/de.toml

@@ -41,3 +41,5 @@ token_expired_title = "Claude Code-Sitzung abgelaufen"
 token_expired_body = "Melde dich erneut an, um die Nutzung weiter zu verfolgen."
 chatgpt_token_expired_title = "Codex-Sitzung abgelaufen"
 chatgpt_token_expired_body = "Melde dich erneut an, um die Nutzung weiter zu verfolgen."
+threshold_80_body = "5-Stunden-Limit naht."
+threshold_95_body = "Limit fast erreicht — gönn dir eine Pause."

src/i18n/locales/en.toml

@@ -41,3 +41,5 @@ token_expired_title = "Claude Code session expired"
 token_expired_body = "Sign in again to keep tracking your usage."
 chatgpt_token_expired_title = "Codex session expired"
 chatgpt_token_expired_body = "Sign in again to keep tracking your usage."
+threshold_80_body = "Approaching the 5-hour limit."
+threshold_95_body = "Limit is close — consider easing up."

src/i18n/locales/es.toml

@@ -41,3 +41,5 @@ token_expired_title = "Sesión de Claude Code caducada"
 token_expired_body = "Vuelve a iniciar sesión para seguir registrando el uso."
 chatgpt_token_expired_title = "Sesión de Codex caducada"
 chatgpt_token_expired_body = "Vuelve a iniciar sesión para seguir registrando el uso."
+threshold_80_body = "Cerca del límite de 5 horas."
+threshold_95_body = "Límite casi alcanzado — reduce el ritmo."

src/i18n/locales/fr.toml

@@ -41,3 +41,5 @@ token_expired_title = "Session Claude Code expirée"
 token_expired_body = "Reconnectez-vous pour continuer à suivre votre utilisation."
 chatgpt_token_expired_title = "Session Codex expirée"
 chatgpt_token_expired_body = "Reconnectez-vous pour continuer à suivre votre utilisation."
+threshold_80_body = "Approche de la limite de 5 heures."
+threshold_95_body = "Limite proche — pensez à lever le pied."

src/i18n/locales/ja.toml

@@ -41,3 +41,5 @@ token_expired_title = "Claude Codeのセッションが切れました"
 token_expired_body = "使用状況の追跡を続けるには再度サインインしてください。"
 chatgpt_token_expired_title = "Codexのセッションが切れました"
 chatgpt_token_expired_body = "使用状況の追跡を続けるには再度サインインしてください。"
+threshold_80_body = "5時間の上限に近づいています。"
+threshold_95_body = "上限に近づきました — ペースを落としましょう。"

src/i18n/locales/ko.toml

@@ -41,3 +41,5 @@ token_expired_title = "Claude Code 세션 만료"
 token_expired_body = "사용량을 계속 추적하려면 다시 로그인하세요."
 chatgpt_token_expired_title = "Codex 세션 만료"
 chatgpt_token_expired_body = "사용량을 계속 추적하려면 다시 로그인하세요."
+threshold_80_body = "5시간 한도에 가까워지고 있어요."
+threshold_95_body = "한도 임박 — 잠시 쉬어가세요."

src/i18n/locales/nl.toml

@@ -41,3 +41,5 @@ token_expired_title = "Claude Code-sessie verlopen"
 token_expired_body = "Meld je opnieuw aan om gebruik te blijven volgen."
 chatgpt_token_expired_title = "Codex-sessie verlopen"
 chatgpt_token_expired_body = "Meld je opnieuw aan om gebruik te blijven volgen."
+threshold_80_body = "Je nadert de 5-uurslimiet."
+threshold_95_body = "Limiet bijna bereikt — overweeg even gas terug te nemen."

src/i18n/locales/zh-TW.toml

@@ -41,3 +41,5 @@ token_expired_title = "Claude Code 工作階段已過期"
 token_expired_body = "請重新登入以繼續追蹤使用量。"
 chatgpt_token_expired_title = "Codex 工作階段已過期"
 chatgpt_token_expired_body = "請重新登入以繼續追蹤使用量。"
+threshold_80_body = "接近 5 小時上限。"
+threshold_95_body = "上限將至 — 建議稍作休息。"

src/i18n/mod.rs

@@ -61,6 +61,12 @@ pub struct LocaleStrings {
     pub token_expired_body: String,
     pub chatgpt_token_expired_title: String,
     pub chatgpt_token_expired_body: String,
+    /// Body text for "your usage just crossed 80% of the 5h limit". The
+    /// title is composed from the provider label + percent so it does not
+    /// need to be translated separately.
+    pub threshold_80_body: String,
+    /// Body text for the 95% threshold balloon.
+    pub threshold_95_body: String,
 }
 
 #[derive(Deserialize)]