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>

frontend/src/pages/DashboardPage.jsx

@@ -212,7 +212,8 @@ export default function DashboardPage() {
         <Link to="/upload" className="text-sm text-blue-400 hover:text-blue-300 transition-colors">+ Import data</Link>
       </div>
 
-      <div className="grid grid-cols-2 lg:grid-cols-4 gap-3">
+      <div className="grid grid-cols-2 lg:grid-cols-5 gap-3">
+        <StatCard label="Steps today" value={health.steps != null ? health.steps.toLocaleString() : '--'} accent="green" sub="goal 10,000" />
         <StatCard label="Running this year" value={ytdStats ? `${ytdStats.running_km.toFixed(0)} km` : '--'} accent="blue" />
         <StatCard label="Cycling this year" value={ytdStats ? `${ytdStats.cycling_km.toFixed(0)} km` : '--'} accent="orange" />
         <StatCard label="Resting HR" value={formatHeartRate(health.resting_hr)} accent="red" />