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.tsx

@@ -18,6 +18,7 @@ export default function App() {
         <Route index element={<DashPage />} />
         <Route path="/dash" element={<DashPage />} />
         <Route path="/activity" element={<TimelineHome />} />
+        <Route path="/activity/:timespan" element={<TimelineHome />} />
         <Route path="/project/:source/*" element={<ProjectPage />} />
         <Route path="/cv" element={<CvPage />} />
       </Route>