feat(ui): contribution graph with daily activity heatmap

Add /v1/activity/daily endpoint returning per-day event counts via
generate_series + LEFT JOIN. Frontend renders an SVG contribution
graph with circles colored by quantile-based thresholds. Clicking a
day navigates to /activity/YYYY-MM-DD showing that day's events.

New /activity/:timespan route parses single dates (YYYY-MM-DD) and
ranges (YYYY-MM-DD..YYYY-MM-DD) from the URL to initialize the
activity timeline filter.

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

ui/src/App.css

@@ -77,6 +77,23 @@ a.hot-pink {
   opacity: 0.3;
 }
 
+.graph-label {
+  fill: #ecf0f1;
+  font-size: 9px;
+  opacity: 0.6;
+}
+
+.graph-cell {
+  cursor: pointer;
+  transition: opacity 0.15s;
+}
+
+.graph-cell:hover {
+  opacity: 0.8;
+  stroke: #ecf0f1;
+  stroke-width: 1;
+}
+
 .project-card {
   background: rgba(255, 255, 255, 0.05);
   border: 1px solid rgba(255, 255, 255, 0.1);