feat(ui): all-time weekly contribution graph + date range timespan support

Add AllTimeGraph component showing one circle per week across the full
history (earliest event to today). Uses the /sources endpoint to find
the earliest date, then fetches daily counts and aggregates to weekly.
Clicking a week navigates to /activity/YYYY-MM-DD..YYYY-MM-DD.

Update parseTimespan to handle both date-only (YYYY-MM-DD) and full
ISO datetime strings in range expressions.

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

ui/src/pages/DashPage.tsx

@@ -4,7 +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';
+import { ContributionGraph, AllTimeGraph } from '../components/ContributionGraph';
 
 export function DashPage() {
   const projectsQ = useQuery({
@@ -27,6 +27,7 @@ export function DashPage() {
         </Col>
       </Row>
       <ContributionGraph />
+      <AllTimeGraph />
       {projectsQ.isLoading && <p>loading...</p>}
       {projectsQ.isError && (
         <p>error: {(projectsQ.error as Error).message}</p>