feat: Phase 4 — MapLibre route map + SVG elevation chart on activity screen

- Add /api/activity/{id}/geojson and /api/activity/{id}/timeseries endpoints
  (bearer-token-gated, falls back from _merged to raw activities dir)
- Rewrite activity detail screen with MapLibreGL v11 API (Map, Camera,
  GeoJSONSource, Layer) and react-native-svg area chart with gradient fill
- On-demand fetch for remote activities that have no local geojson/timeseries
- Add react-native-svg dependency; requires dev build (npx expo run:android)

bincio/serve/server.py

@@ -455,6 +455,38 @@ async def stats() -> JSONResponse:
     })
 
 
+@app.get("/api/activity/{activity_id}/geojson")
+async def get_activity_geojson(
+    activity_id: str,
+    user: User = Depends(_require_auth),
+) -> JSONResponse:
+    """Return GeoJSON track for an activity (mobile detail screen)."""
+    _check_id(activity_id)
+    dd = _get_data_dir()
+    user_dir = dd / user.handle
+    for base in (user_dir / "_merged" / "activities", user_dir / "activities"):
+        p = base / f"{activity_id}.geojson"
+        if p.exists():
+            return JSONResponse(json.loads(p.read_text()))
+    raise HTTPException(404, "GeoJSON not found")
+
+
+@app.get("/api/activity/{activity_id}/timeseries")
+async def get_activity_timeseries(
+    activity_id: str,
+    user: User = Depends(_require_auth),
+) -> JSONResponse:
+    """Return timeseries for an activity (mobile detail screen)."""
+    _check_id(activity_id)
+    dd = _get_data_dir()
+    user_dir = dd / user.handle
+    for base in (user_dir / "_merged" / "activities", user_dir / "activities"):
+        p = base / f"{activity_id}.timeseries.json"
+        if p.exists():
+            return JSONResponse(json.loads(p.read_text()))
+    raise HTTPException(404, "Timeseries not found")
+
+
 @app.get("/api/wheel/version")
 async def wheel_version() -> JSONResponse:
     """Public endpoint: current bincio wheel version for mobile app update checks."""