Auto-fetch wind, drop manual fetch/validate buttons, fix it in fly mode

Redesigns the waypoint list panel to match the drone-status/missions
panels: no title bar, close button floats over the tab row, tabs get
more vertical padding.

Removes the "Fetch wind" and "Validate" header buttons along with the
now-fully-orphaned WindValidatePanel screen and its WindService
support code (fetchValidation, WindValidationResult and friends) -
nothing else referenced them.

Wind is now fetched automatically instead: HeaderWindNotifier gains
ensurePerWaypointWind(), called when the waypoint list panel opens and
when the header pill's per-waypoint toggle switches on. A lat/lon
signature of the waypoint list tracks whether the fetched data is
still current, so a ref.listen(currentMissionProvider) in the same
notifier refetches only when the list itself changes (position added/
removed/moved) - not on every altitude/speed edit. Waypoints that
already carry wind data (e.g. loaded from a saved mission) short-
circuit the check instead of re-fetching pointlessly.

Also wires per-waypoint wind markers into the fly-mode map, which
never had them (plan_screen.dart built windMarkers; fly_screen.dart
just never did) - the reported "doesn't work in fly mode" bug.
windMarkerWidth() moved from a plan_screen.dart-private method to
wind_marker_pill.dart so both screens can share it.

Test containers in widget_test.dart now override windServiceProvider
with a MockClient so the new auto-fetch trigger can't reach the real
network during tests.

app/lib/ui/screens/plan/plan_screen.dart

@@ -432,7 +432,7 @@ class _PlanScreenState extends ConsumerState<PlanScreen> {
                 if (wp.windSpeedMs != null && wp.windDirFromDeg != null)
                   Marker(
                     point: LatLng(wp.lat, wp.lon),
-                    width: _windMarkerWidth(wp.windSpeedMs!),
+                    width: windMarkerWidth(wp.windSpeedMs!),
                     height: 22,
                     alignment: const Alignment(1.1, -1.5),
                     child: WindMarkerPill(
@@ -556,19 +556,4 @@ class _PlanScreenState extends ConsumerState<PlanScreen> {
       borderColor: Colors.white,
     );
   }
-
-  /// flutter_map's Marker verlangt eine feste Breite (kein intrinsisches
-  /// Sizing wie bei normalen Widgets) - eine einzelne fuer alle Faelle
-  /// grosszuegig bemessene Breite liesse bei kurzen Werten (z.B. "1 m/s")
-  /// sichtbaren Leerraum rechts neben dem Text. Stattdessen je Ziffernzahl
-  /// berechnet (per Widget-Messung ermittelt: +11px/Ziffer, siehe
-  /// WindMarkerPill).
-  double _windMarkerWidth(double speedMs) {
-    final digits = speedMs.round().abs().toString().length;
-    // +1px Puffer: MarkerLayer rendert ueber eine eigene Transform-/
-    // OverflowBox-Kette, die im echten Baum minimal (< 1px) anders rundet
-    // als eine isolierte Messung - ohne Puffer fuehrte das zu einem
-    // 0.25px-Overflow.
-    return 81.0 + digits * 11.0;
-  }
 }