Add segments, YTD stats, route matching fixes, body battery layout, pace fix

- Segments page: new /segments route with auto-generate (1km splits, turn
  detection, hill detection), manual segment creation, per-segment performance
  times across matched activities; fixed auth on existing segment endpoints
- YTD distance: new /activities/stats/ytd endpoint; Dashboard replaces
  'Total distance' with 'Running this year' + 'Cycling this year'; Activities
  page shows YTD stats row
- Weekly chart click: clicking a Dashboard bar navigates to Activities filtered
  to that week; Activities reads from/to query params with dismissable chip
- Route matching: add ±2.5% distance gate + 3% relative DTW threshold
  (was flat 80m); tighten candidate pre-filter from 80/120% to 95/105%
- Body battery layout: HR chart and body battery now side-by-side at same
  height on large screens instead of stacked full-width
- Pace display fix: MetricTimeline clamps GPS speed outliers before computing
  Y-axis domain; tick formatter guards against v<=0 or v>25 m/s

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -8,6 +8,7 @@ import ActivitiesPage from './pages/ActivitiesPage'
 import ActivityDetailPage from './pages/ActivityDetailPage'
 import HealthPage from './pages/HealthPage'
 import RoutesPage from './pages/RoutesPage'
+import SegmentsPage from './pages/SegmentsPage'
 import RecordsPage from './pages/RecordsPage'
 import UploadPage from './pages/UploadPage'
 import ProfilePage from './pages/ProfilePage'
@@ -34,6 +35,7 @@ export default function App() {
         <Route path="activities/:id" element={<ActivityDetailPage />} />
         <Route path="health" element={<HealthPage />} />
         <Route path="routes" element={<RoutesPage />} />
+        <Route path="segments" element={<SegmentsPage />} />
         <Route path="records" element={<RecordsPage />} />
         <Route path="upload" element={<UploadPage />} />
         <Route path="profile" element={<ProfilePage />} />