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/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 => {