Add trend-range gating, vehicle filter, sync cancel, moving time, and UI fixes

- Grey out trend ranges beyond available health history
- Reject implausibly fast (vehicle) activities on upload with feedback
- Add cancel button + cooperative cancellation for Garmin sync
- Show daily steps prominently on the dashboard
- Clear errors for malformed/empty upload ZIPs
- Snap-target dot when drawing a segment on the map
- Time-axis fallback for stationary/HIIT HR timelines; hide map when no GPS
- Parse and display moving time (timer) vs elapsed; backfill task
- Restyle SegmentsPanel like RouteLeaderboard; Laps/Routes/Segments on one row

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/activities.py

@@ -35,6 +35,7 @@ class ActivitySummary(BaseModel):
 
 class ActivityDetail(ActivitySummary):
     end_time: Optional[datetime]
+    moving_time_s: Optional[float]
     elevation_loss_m: Optional[float]
     max_heart_rate: Optional[float]
     avg_power: Optional[float]