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/models/user.py

@@ -92,7 +92,8 @@ class Activity(Base):
     start_time = Column(DateTime(timezone=True), nullable=False, index=True)
     end_time = Column(DateTime(timezone=True), nullable=True)
     distance_m = Column(Float, nullable=True)
-    duration_s = Column(Float, nullable=True)
+    duration_s = Column(Float, nullable=True)          # total elapsed time (wall clock, incl. pauses)
+    moving_time_s = Column(Float, nullable=True)        # timer time — excludes paused periods
     elevation_gain_m = Column(Float, nullable=True)
     elevation_loss_m = Column(Float, nullable=True)
     avg_heart_rate = Column(Float, nullable=True)