refactor: match github-profile-summary-cards chart styles

- Productive time is now a 24-hour bar chart with axes and nice tick labels
  instead of a 7x24 heatmap. Model Productive field reshaped from
  [7][24]int to [24]int.
- Language cards render as donut charts with a left-side legend instead of
  a stacked bar. Slices beyond top-6 collapse into an "Other" row.
- Add niceTicks helper (1/2/5 * 10^k ladder, d3-style) for axis ticks.
- Legacy language_bar.go removed.

internal/github/model.go

@@ -35,8 +35,8 @@ type Profile struct {
 	// primary language, sorted desc. Populated by FetchProductive.
 	CommitsByLanguage []LangStat
 
-	// Commit-count histogram indexed by [day-of-week 0=Sunday][hour-of-day 0-23].
-	Productive [7][24]int
+	// Commit counts grouped by hour-of-day (0-23) in the configured timezone.
+	Productive [24]int
 
 	// TopRepos are owned repos sorted by stargazer count desc. Populated by
 	// FetchProfile and consumed by FetchProductive.

internal/github/productive.go

@@ -23,7 +23,7 @@ type productiveGQL struct {
 	} `json:"repository"`
 }
 
-// FetchProductive fills p.Productive with a [7][24] commit histogram over the
+// FetchProductive fills p.Productive with a 24-hour commit histogram over the
 // last year and p.CommitsByLanguage with commit counts attributed to each
 // repo's primary language. Commits are gathered from the given repos (usually
 // p.TopRepos[:N]); each repo is sampled up to maxPerRepo commits to keep the
@@ -71,8 +71,7 @@ func (c *Client) FetchProductive(p *Profile, repos []RepoInfo, loc *time.Locatio
 				if err != nil {
 					continue
 				}
-				tl := t.In(loc)
-				p.Productive[int(tl.Weekday())][tl.Hour()]++
+				p.Productive[t.In(loc).Hour()]++
 				if repo.PrimaryLanguage != "" {
 					commitsByLang[repo.PrimaryLanguage]++
 					if _, ok := langColor[repo.PrimaryLanguage]; !ok {