feat: implement profile summary cards with GraphQL fetch and Action wrapper

- Add GraphQL client fetching profile, stats, language aggregation, and
  per-repo commit histograms for the productive-time heatmap.
- Render real SVG cards (profile details, top languages, stats grid,
  weekday×hour heatmap) with XML escaping and thousands-formatted numbers.
- Expand theme palette to 30 built-ins ported from github-readme-stats;
  add -list-themes, multi-theme rendering, and 'all' shortcut.
- Package as Docker-based GitHub Action (action.yml, Dockerfile,
  entrypoint.sh) with optional auto-commit of generated cards.
- Release workflow publishes GHCR image and cross-platform binaries on
  v* tags.
- Unit tests cover rendering, XML escape, number formatting, language sort.

internal/card/productive.go

@@ -2,6 +2,7 @@ package card
 
 import (
 	"fmt"
+	"strings"
 
 	"github.com/tiennm99/ghstats/internal/github"
 	"github.com/tiennm99/ghstats/internal/theme"
@@ -11,13 +12,71 @@ type productiveCard struct{}
 
 func (productiveCard) Filename() string { return "3-productive-time.svg" }
 
+var weekdayLabels = [7]string{"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}
+
 func (productiveCard) SVG(p *github.Profile, t theme.Theme) ([]byte, error) {
-	// TODO: render the [7][24]int heatmap from p.Productive.
-	svg := fmt.Sprintf(`<svg xmlns="http://www.w3.org/2000/svg" width="400" height="200">
-  <rect width="100%%" height="100%%" fill="%s"/>
-  <text x="20" y="40" fill="%s" font-family="sans-serif" font-size="20">Productive Time</text>
-  <text x="20" y="80" fill="%s" font-family="sans-serif" font-size="12">Heatmap placeholder</text>
-</svg>`, t.Background, t.Title, t.Text)
-	_ = p
-	return []byte(svg), nil
+	const (
+		width    = 650
+		height   = 240
+		cellSize = 18
+		cellGap  = 3
+		gridX    = 55
+		gridY    = 60
+	)
+
+	var b strings.Builder
+	b.WriteString(header(width, height, t.Background, t.Title, "Productive Time (last year, by hour)"))
+
+	max := 0
+	for _, row := range p.Productive {
+		for _, v := range row {
+			if v > max {
+				max = v
+			}
+		}
+	}
+
+	// Weekday labels.
+	for i, d := range weekdayLabels {
+		y := gridY + i*(cellSize+cellGap) + cellSize - 4
+		fmt.Fprintf(&b, `
+  <text x="25" y="%d" font-size="11" fill="%s">%s</text>`,
+			y, t.Muted, d)
+	}
+
+	// Hour labels along top (every 3 hours).
+	for h := 0; h < 24; h += 3 {
+		x := gridX + h*(cellSize+cellGap)
+		fmt.Fprintf(&b, `
+  <text x="%d" y="55" font-size="10" fill="%s">%02dh</text>`,
+			x, t.Muted, h)
+	}
+
+	// Cells.
+	for d := 0; d < 7; d++ {
+		for h := 0; h < 24; h++ {
+			count := p.Productive[d][h]
+			opacity := heatOpacity(count, max)
+			x := gridX + h*(cellSize+cellGap)
+			y := gridY + d*(cellSize+cellGap)
+			fmt.Fprintf(&b, `
+  <rect x="%d" y="%d" width="%d" height="%d" rx="3" fill="%s" fill-opacity="%.2f"><title>%s %02d:00 — %d commits</title></rect>`,
+				x, y, cellSize, cellSize, t.Accent, opacity,
+				weekdayLabels[d], h, count)
+		}
+	}
+
+	b.WriteString(footer)
+	return []byte(b.String()), nil
+}
+
+// heatOpacity returns the fill-opacity for a cell. Zero is almost transparent
+// so the grid is still visible; max-count maps to fully opaque.
+func heatOpacity(count, max int) float64 {
+	if max == 0 {
+		return 0.08
+	}
+	const floor = 0.10
+	ratio := float64(count) / float64(max)
+	return floor + (1.0-floor)*ratio
 }