refactor(theme): port full github-profile-summary-cards palette

- Replace the previous 30-theme list with the complete 61-theme palette
  from github-profile-summary-cards (snake_case ids, e.g. github_dark,
  nord_bright, solarized_dark).
- Add Stroke and StrokeOpacity fields to Theme; card frame now uses the
  theme's own stroke instead of a hardcoded white overlay so light themes
  render a visible border.
- Drop the github-readme-stats credit from README; all palette data now
  comes from one upstream source.

internal/card/donut_chart.go

@@ -30,7 +30,7 @@ func renderDonutCard(title string, stats []github.LangStat, t theme.Theme) []byt
 	stats = collapseOther(stats, topN)
 
 	var b strings.Builder
-	b.WriteString(header(width, height, t.Background, t.Title, title))
+	b.WriteString(header(width, height, t.Background, t.Stroke, t.StrokeOpacity, t.Title, title))
 
 	if len(stats) == 0 {
 		fmt.Fprintf(&b, `