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/card/card.go

@@ -26,6 +26,7 @@ var allCards = []Card{
 	mostCommitLanguageCard{},
 	statsCard{},
 	productiveCard{},
+	contributionsCard{},
 }
 
 // RenderAll writes every card into outDir/<themeID>/.

internal/card/contributions.go

@@ -0,0 +1,186 @@
+package card
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/tiennm99/ghstats/internal/github"
+	"github.com/tiennm99/ghstats/internal/theme"
+)
+
+type contributionsCard struct{}
+
+func (contributionsCard) Filename() string { return "5-contributions.svg" }
+
+// monthBucket holds a calendar month's aggregate contribution count.
+type monthBucket struct {
+	Year  int
+	Month time.Month
+	Count int
+}
+
+func (contributionsCard) SVG(p *github.Profile, t theme.Theme) ([]byte, error) {
+	const (
+		width    = 500
+		height   = 220
+		leftPad  = 35
+		rightPad = 35
+		topPad   = 60
+		chartH   = 120
+	)
+	chartW := width - leftPad - rightPad
+
+	var b strings.Builder
+	b.WriteString(header(width, height, t.Background, t.Stroke, t.StrokeOpacity, t.Title, "Contributions (last year)"))
+
+	buckets := aggregateByMonth(p.DailyContributions)
+	if len(buckets) < 2 {
+		fmt.Fprintf(&b, `
+  <text x="25" y="90" font-size="13" fill="%s">No contribution data available.</text>`, t.Muted)
+		b.WriteString(footer)
+		return []byte(b.String()), nil
+	}
+
+	// Y scale based on max monthly count; nice ticks for labels.
+	var maxVal int
+	for _, bk := range buckets {
+		if bk.Count > maxVal {
+			maxVal = bk.Count
+		}
+	}
+	yMax := float64(maxVal)
+	if yMax == 0 {
+		yMax = 1
+	}
+	ticks := niceTicks(yMax, 5)
+	if len(ticks) > 0 {
+		yMax = ticks[len(ticks)-1]
+	}
+
+	// Map each bucket to an (x, y) point on the chart grid.
+	pts := make([][2]float64, len(buckets))
+	for i, bk := range buckets {
+		x := float64(leftPad) + float64(chartW)*float64(i)/float64(len(buckets)-1)
+		y := float64(topPad+chartH) - float64(chartH)*float64(bk.Count)/yMax
+		pts[i] = [2]float64{x, y}
+	}
+
+	// Y axis mirrored on both sides with matching nice ticks.
+	rightX := leftPad + chartW
+	fmt.Fprintf(&b, `
+  <line x1="%d" y1="%d" x2="%d" y2="%d" stroke="%s"/>
+  <line x1="%d" y1="%d" x2="%d" y2="%d" stroke="%s"/>`,
+		leftPad, topPad, leftPad, topPad+chartH, t.Muted,
+		rightX, topPad, rightX, topPad+chartH, t.Muted)
+	for _, v := range ticks {
+		y := topPad + chartH - int(float64(chartH)*v/yMax)
+		label := escapeXML(formatTick(v))
+		fmt.Fprintf(&b, `
+  <line x1="%d" y1="%d" x2="%d" y2="%d" stroke="%s"/>
+  <text x="%d" y="%d" font-size="10" fill="%s" text-anchor="end">%s</text>
+  <line x1="%d" y1="%d" x2="%d" y2="%d" stroke="%s"/>
+  <text x="%d" y="%d" font-size="10" fill="%s" text-anchor="start">%s</text>`,
+			leftPad-4, y, leftPad, y, t.Muted,
+			leftPad-6, y+3, t.Muted, label,
+			rightX, y, rightX+4, y, t.Muted,
+			rightX+6, y+3, t.Muted, label)
+	}
+
+	// X axis baseline + month labels (every other month to avoid overlap).
+	fmt.Fprintf(&b, `
+  <line x1="%d" y1="%d" x2="%d" y2="%d" stroke="%s"/>`,
+		leftPad, topPad+chartH, leftPad+chartW, topPad+chartH, t.Muted)
+	for i, bk := range buckets {
+		if i%2 != 0 && i != len(buckets)-1 {
+			continue
+		}
+		x := int(pts[i][0])
+		label := fmt.Sprintf("%02d/%02d", bk.Year%100, int(bk.Month))
+		fmt.Fprintf(&b, `
+  <line x1="%d" y1="%d" x2="%d" y2="%d" stroke="%s"/>
+  <text x="%d" y="%d" font-size="10" fill="%s" text-anchor="middle">%s</text>`,
+			x, topPad+chartH, x, topPad+chartH+4, t.Muted,
+			x, topPad+chartH+16, t.Muted, label)
+	}
+
+	// Smooth filled area using Catmull-Rom → cubic Bezier segments.
+	path := catmullRomAreaPath(pts, float64(topPad+chartH))
+	fmt.Fprintf(&b, `
+  <path d="%s" fill="%s" fill-opacity="0.25" stroke="none"/>
+  <path d="%s" fill="none" stroke="%s" stroke-width="2"/>`,
+		path, t.Accent,
+		catmullRomLinePath(pts), t.Accent)
+
+	b.WriteString(footer)
+	return []byte(b.String()), nil
+}
+
+// aggregateByMonth bins the daily series into consecutive month buckets
+// sorted oldest→newest. Empty months between first and last are kept as
+// zero-count rows so the area chart remains time-continuous.
+func aggregateByMonth(days []github.DailyContribution) []monthBucket {
+	if len(days) == 0 {
+		return nil
+	}
+	counts := map[string]int{}
+	for _, d := range days {
+		key := fmt.Sprintf("%04d-%02d", d.Date.Year(), int(d.Date.Month()))
+		counts[key] += d.Count
+	}
+
+	// Walk from the first to last month of the calendar inclusively.
+	first := days[0].Date
+	last := days[len(days)-1].Date
+	cur := time.Date(first.Year(), first.Month(), 1, 0, 0, 0, 0, time.UTC)
+	end := time.Date(last.Year(), last.Month(), 1, 0, 0, 0, 0, time.UTC)
+
+	var out []monthBucket
+	for !cur.After(end) {
+		key := fmt.Sprintf("%04d-%02d", cur.Year(), int(cur.Month()))
+		out = append(out, monthBucket{
+			Year:  cur.Year(),
+			Month: cur.Month(),
+			Count: counts[key],
+		})
+		cur = cur.AddDate(0, 1, 0)
+	}
+	return out
+}
+
+// catmullRomLinePath produces an SVG path string that passes through each
+// point using cubic Bezier segments derived from the Catmull-Rom spline
+// (tension = 0.5, the classic d3.curveCatmullRom default).
+func catmullRomLinePath(pts [][2]float64) string {
+	if len(pts) < 2 {
+		return ""
+	}
+	var b strings.Builder
+	fmt.Fprintf(&b, "M%.2f,%.2f", pts[0][0], pts[0][1])
+	for i := 0; i < len(pts)-1; i++ {
+		p0 := pts[max(i-1, 0)]
+		p1 := pts[i]
+		p2 := pts[i+1]
+		p3 := pts[min(i+2, len(pts)-1)]
+
+		c1x := p1[0] + (p2[0]-p0[0])/6
+		c1y := p1[1] + (p2[1]-p0[1])/6
+		c2x := p2[0] - (p3[0]-p1[0])/6
+		c2y := p2[1] - (p3[1]-p1[1])/6
+
+		fmt.Fprintf(&b, " C%.2f,%.2f %.2f,%.2f %.2f,%.2f",
+			c1x, c1y, c2x, c2y, p2[0], p2[1])
+	}
+	return b.String()
+}
+
+// catmullRomAreaPath closes the smooth line path down to the baseline so it
+// can be filled as an area under the curve.
+func catmullRomAreaPath(pts [][2]float64, baseline float64) string {
+	line := catmullRomLinePath(pts)
+	if line == "" {
+		return ""
+	}
+	return fmt.Sprintf("%s L%.2f,%.2f L%.2f,%.2f Z",
+		line, pts[len(pts)-1][0], baseline, pts[0][0], baseline)
+}

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 {

internal/github/profile.go

@@ -36,6 +36,12 @@ type profileGQL struct {
 			RestrictedContributionsCount        int `json:"restrictedContributionsCount"`
 			ContributionCalendar                struct {
 				TotalContributions int `json:"totalContributions"`
+				Weeks              []struct {
+					ContributionDays []struct {
+						ContributionCount int    `json:"contributionCount"`
+						Date              string `json:"date"`
+					} `json:"contributionDays"`
+				} `json:"weeks"`
 			} `json:"contributionCalendar"`
 		} `json:"contributionsCollection"`
 
@@ -101,6 +107,20 @@ func (c *Client) FetchProfile(login string) (*Profile, error) {
 			p.TotalCommits = cc.TotalCommitContributions
 			p.TotalReviews = cc.TotalPullRequestReviewContributions
 			p.TotalContributions = cc.ContributionCalendar.TotalContributions + cc.RestrictedContributionsCount
+
+			// Flatten week → day into a linear daily series sorted by date.
+			for _, w := range cc.ContributionCalendar.Weeks {
+				for _, d := range w.ContributionDays {
+					t, err := time.Parse("2006-01-02", d.Date)
+					if err != nil {
+						continue
+					}
+					p.DailyContributions = append(p.DailyContributions, DailyContribution{
+						Date:  t,
+						Count: d.ContributionCount,
+					})
+				}
+			}
 		}
 
 		for _, r := range u.Repositories.Nodes {

internal/github/queries.go

@@ -30,7 +30,15 @@ query($login: String!, $after: String) {
       totalPullRequestReviewContributions
       totalRepositoryContributions
       restrictedContributionsCount
-      contributionCalendar { totalContributions }
+      contributionCalendar {
+        totalContributions
+        weeks {
+          contributionDays {
+            contributionCount
+            date
+          }
+        }
+      }
     }
     repositories(
       first: 100