Segments Phase 4: detail page, activity efforts, athlete tab, new APIs

New API endpoints:
- GET /api/segments/{id} — single segment metadata
- GET /api/activities/{id}/segment_efforts — efforts for an activity (auth)
- GET /api/users/{handle}/segment_summary — public best time + count per segment

New components:
- SegmentDetail.svelte — map + metadata + effort table (with PR/Δ) + rescan button
- SegmentsPage.svelte — URL router: shows detail when /segments/{id}/, list otherwise

Updated:
- segments/index.astro — now uses SegmentsPage router
- nginx-activity.conf — add /segments/ try_files rule for client-side routing
- ActivityDetail.svelte — segment efforts block below laps
- AthleteView.svelte — Segments tab with best time + effort count per segment
- format.ts — add formatElapsed() for compact m:ss display

site/src/pages/u/[handle]/athlete/index.astro

@@ -36,7 +36,7 @@ const athleteUrl = `${mergedBase}athlete.json`;
       </a>
     </div>
   </div>
-  <AthleteView {base} {indexUrl} {athleteUrl} client:only="svelte" />
+  <AthleteView {base} {indexUrl} {athleteUrl} {handle} client:only="svelte" />
 </Base>
 <script define:vars={{ handle }}>
   function applyMeCheck(me) {