feat: bulk delete + merge activities in feed

- Select mode in ActivityFeed: toggle with Select button (logged-in only),
  cards become clickable with checkmark indicator, action bar fixed at bottom
- Bulk delete: calls existing DELETE /api/activity/{id} for each selected,
  removes from local feed state immediately
- Bulk merge: POST /api/merge sorts by started_at (earliest = primary),
  sums distance/duration/elevation, weighted-averages HR/power, concatenates
  geojson and timeseries; backs up originals to _merge_backup/ for recovery
- GET /api/merges returns per-user hidden list; feed filters secondaries
  client-side on load so static shards don't need a rebuild to hide them
- POST /api/unmerge/{id} restores primary from backup, unhides secondaries
- ActivityDetail: shows "Merged (N)" badge + Unmerge button for owners
- Fix: edit button now works from personal profile feed (handle was missing
  from year-shard activities; injected from filterHandle on sessionStorage write)

bincio/serve/server.py

@@ -20,12 +20,14 @@ from bincio.serve.routers import (
     activities,
     admin,
     auth,
+    budget,
     download,
     feed,
     garmin,
     gear,
     ideas,
     me,
+    merge,
     ogimage,
     segments,
     strava,
@@ -63,6 +65,8 @@ for _router in [
     me.router,
     admin.router,
     activities.router,
+    merge.router,
+    budget.router,
     download.router,
     uploads.router,
     segments.router,