fix(ui): scale contribution graph to full container width

Use viewBox + width=100% instead of fixed pixel dimensions so the
SVG scales to match the project card grid below.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ContributionGraph.tsx

@@ -93,8 +93,8 @@ export function ContributionGraph() {
       <p style={{ fontSize: '0.8rem', opacity: 0.6 }}>
         {totalCount} contributions in the last year
       </p>
-      <div style={{ overflowX: 'auto' }}>
+      <div>
-        <svg width={svgWidth} height={svgHeight} className="d-block">
+        <svg viewBox={`0 0 ${svgWidth} ${svgHeight}`} width="100%" className="d-block">
           {/* Day-of-week labels */}
           {DAY_LABELS.map((label, i) =>
             label ? (