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/components/ui/Layout.jsx

@@ -6,6 +6,7 @@ const nav = [
   { to: '/activities', label: 'Activities', icon: '🏃' },
   { to: '/health', label: 'Health', icon: '❤️' },
   { to: '/routes', label: 'Routes', icon: '🗺️' },
+  { to: '/segments', label: 'Segments', icon: '📏' },
   { to: '/records', label: 'Records', icon: '🏆' },
   { to: '/upload', label: 'Import', icon: '⬆️' },
   { to: '/profile', label: 'Profile', icon: '⚙️' },