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/main.py

@@ -50,6 +50,15 @@ async def init_db():
     except Exception as e:
         print(f"Column migration skipped: {e}")
 
+    # activities.moving_time_s column added after initial creation (timer time)
+    try:
+        async with engine.begin() as conn:
+            await conn.execute(text(
+                "ALTER TABLE activities ADD COLUMN IF NOT EXISTS moving_time_s FLOAT"
+            ))
+    except Exception as e:
+        print(f"activities.moving_time_s column migration skipped: {e}")
+
     # health_metrics columns added after initial creation
     try:
         async with engine.begin() as conn: