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/pages/DashPage.tsx

@@ -4,6 +4,7 @@ import Col from 'react-bootstrap/Col';
 import Row from 'react-bootstrap/Row';
 
 import { fetchProjects, fetchLanguages, type ProjectSummary, type Source } from '../api/client';
+import { ContributionGraph } from '../components/ContributionGraph';
 
 export function DashPage() {
   const projectsQ = useQuery({
@@ -25,6 +26,7 @@ export function DashPage() {
           </p>
         </Col>
       </Row>
+      <ContributionGraph />
       {projectsQ.isLoading && <p>loading...</p>}
       {projectsQ.isError && (
         <p>error: {(projectsQ.error as Error).message}</p>