fix(contributions): trim last-year series to clean 13-month span

GitHub's contributionCalendar is week-aligned, so a "last year" query on
2026-05-02 returns days back to ~2025-04-27. That extra partial month
inflated aggregateByMonth to 14 buckets and the x-axis stride logic placed
04/26 and 05/26 next to each other (x=290 / x=312), overlapping.

Trim to start-of-(last.Month - 1y) so the chart spans a clean 13 months
ending at the current month. Heatmap path is untouched — it needs the
full week-aligned series.

internal/card/contributions.go

@@ -14,7 +14,28 @@ type contributionsCard struct{}
 func (contributionsCard) Filename() string { return "contributions.svg" }
 
 func (contributionsCard) SVG(p *github.Profile, t theme.Theme) ([]byte, error) {
-	return renderContributions("Contributions (last year)", p.DailyContributions, t), nil
+	return renderContributions("Contributions (last year)", trimToLastYear(p.DailyContributions), t), nil
+}
+
+// trimToLastYear keeps only the trailing 13-month window ending at the
+// series' last day. GitHub's contributionCalendar is week-aligned, so a
+// "last year" query bleeds into the calendar month before today minus one
+// year (e.g. asking on 2026-05-02 returns days back to ~2025-04-27). That
+// extra partial month inflates the bucket count to 14 and forces the x-axis
+// stride logic to place the final two month labels right next to each other.
+// Trimming to start-of-(today.Month - 1y) restores a clean 13-bucket span.
+func trimToLastYear(days []github.DailyContribution) []github.DailyContribution {
+	if len(days) == 0 {
+		return days
+	}
+	last := days[len(days)-1].Date
+	cutoff := time.Date(last.Year()-1, last.Month(), 1, 0, 0, 0, 0, time.UTC)
+	for i, d := range days {
+		if !d.Date.Before(cutoff) {
+			return days[i:]
+		}
+	}
+	return nil
 }
 
 type contributionsAllTimeCard struct{}

internal/card/contributions_trim_test.go

@@ -0,0 +1,52 @@
+package card
+
+import (
+	"testing"
+	"time"
+
+	"github.com/tiennm99/ghstats/internal/github"
+)
+
+// GitHub's contributionCalendar is week-aligned, so a "last year" series for a
+// query made on 2026-05-02 (Saturday) starts on 2025-04-27 (Sunday). Without
+// trimming the chart shows 14 calendar months and the x-axis label stride
+// places 04/26 and 05/26 next to each other, overlapping. Trimming should
+// drop everything before the first day of last.Month minus one year.
+func TestTrimToLastYearProducesCleanThirteenMonthSpan(t *testing.T) {
+	start := time.Date(2025, 4, 27, 0, 0, 0, 0, time.UTC)
+	end := time.Date(2026, 5, 2, 0, 0, 0, 0, time.UTC)
+	var days []github.DailyContribution
+	for d := start; !d.After(end); d = d.AddDate(0, 0, 1) {
+		days = append(days, github.DailyContribution{Date: d, Count: 1})
+	}
+
+	trimmed := trimToLastYear(days)
+	if len(trimmed) == 0 {
+		t.Fatal("trimToLastYear returned no days")
+	}
+	first := trimmed[0].Date
+	wantFirst := time.Date(2025, 5, 1, 0, 0, 0, 0, time.UTC)
+	if !first.Equal(wantFirst) {
+		t.Fatalf("first day = %s, want %s", first, wantFirst)
+	}
+	if !trimmed[len(trimmed)-1].Date.Equal(end) {
+		t.Fatalf("last day = %s, want %s", trimmed[len(trimmed)-1].Date, end)
+	}
+
+	buckets := aggregateByMonth(trimmed)
+	if len(buckets) != 13 {
+		t.Fatalf("bucket count = %d, want 13 (May 2025 .. May 2026)", len(buckets))
+	}
+	if buckets[0].Year != 2025 || buckets[0].Month != time.May {
+		t.Fatalf("first bucket = %d/%d, want 2025/May", buckets[0].Year, buckets[0].Month)
+	}
+	if buckets[12].Year != 2026 || buckets[12].Month != time.May {
+		t.Fatalf("last bucket = %d/%d, want 2026/May", buckets[12].Year, buckets[12].Month)
+	}
+}
+
+func TestTrimToLastYearEmptyInput(t *testing.T) {
+	if got := trimToLastYear(nil); got != nil {
+		t.Fatalf("trimToLastYear(nil) = %v, want nil", got)
+	}
+}