feat(card): add contributions area chart card

New 5-contributions.svg renders the last year's contribution calendar
as a monthly smooth-filled area chart. Pure Go SVG; no extra API calls
— one additional contributionCalendar.weeks block in the existing
profile GraphQL query carries the data.

- Y-axis mirrored on both sides with nice ticks.
- X-axis labels in YY/MM format, every other month to avoid overlap.
- Smooth curve via Catmull-Rom interpolation converted to cubic Bezier
  (d3.curveCatmullRom default tension 0.5).
- Missing months between first and last are inserted as zero-count so
  the chart stays time-continuous.

internal/github/model.go

@@ -38,11 +38,22 @@ type Profile struct {
 	// Commit counts grouped by hour-of-day (0-23) in the configured timezone.
 	Productive [24]int
 
+	// DailyContributions is the raw per-day contribution calendar covering
+	// the most recent year. The area chart aggregates it into monthly
+	// buckets; kept granular here so any downstream card can re-bin freely.
+	DailyContributions []DailyContribution
+
 	// TopRepos are owned repos sorted by stargazer count desc. Populated by
 	// FetchProfile and consumed by FetchProductive.
 	TopRepos []RepoInfo
 }
 
+// DailyContribution is a single day in the contributions calendar.
+type DailyContribution struct {
+	Date  time.Time
+	Count int
+}
+
 // LangStat is one row in a language breakdown card. Value is repo count or
 // commit count depending on which slice holds it.
 type LangStat struct {