feat(card): add S-tier cards — heatmap, streak, by-year, weekday, top-starred (#3)

Five new cards, all derived from data FetchProductive / FetchProfile already
pull, so zero additional API calls:

- contributions-heatmap: 7×53 calendar grid with a 5-bucket intensity ramp
  mixed from each theme's Background→Accent so palettes with no dedicated
  heat ramp still render sensibly.
- streak: current streak, longest streak with date ranges, active/total days.
- contributions-by-year: one bar per active year, peak year highlighted.
- productive-weekday + -all-time: 7-bar day-of-week mirror of the hour-of-day
  cards; FetchProductive now also fills Weekday / WeekdayAllTime histograms
  during the same commit-history pass.
- top-starred-repos: top 5 owned non-fork repos by stargazer count; threads
  Stars through RepoInfo.

Card count: 9 → 14. Registered in allCards grouped by recency (last-year
block, then all-time block). Render test extended to cover all new files
and realistic daily-series inputs.

internal/github/productive.go

@@ -94,9 +94,11 @@ func (c *Client) FetchProductive(ctx context.Context, p *Profile, repos []RepoIn
 				}
 				tl := t.In(loc)
 				p.ProductiveAllTime[tl.Hour()]++
+				p.WeekdayAllTime[tl.Weekday()]++
 				attributeCommit(repo, repoTotal, allTimeLang, langColor)
 				if tl.After(yearAgo) {
 					p.Productive[tl.Hour()]++
+					p.Weekday[tl.Weekday()]++
 					attributeCommit(repo, repoTotal, lastYearLang, langColor)
 				}
 				seen++