fix: mobile UX — GPS permission flow, traffic toggle, overflow

GPS permission:
- Remove silent mount-time getCurrentPosition calls that cause iOS Safari
  to cache a "denied" state without ever prompting the user
- LocateButton always retries getCurrentPosition on tap (user gesture)
- Only show "denied" toast on PERMISSION_DENIED (code 1), not timeout
- MapView watchPosition now starts only after confirmed grant, not unconditionally

Traffic toggle:
- Fix isStyleLoaded() race in LayerControl — if style not loaded when
  toggle fires, defer to map.once("style.load") instead of silently bailing
- Change outside-click handler from mousedown to pointerdown for mobile

Mobile UX (from prior session):
- Add LocateButton component (crosshair GPS locate/re-center)
- Reposition layer control + locate button to top-right on mobile
  (below MapLibre nav controls, above bottom sheet)
- ModeSelector: add min-w-0 to prevent flex overflow at 390px
- StopItem: remove button visible on touch (60% opacity vs hover-only)
- Panel: overflow-x-hidden + safe-area-inset-bottom on mobile sheet
- Body overflow-x guard on mobile viewports

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/Panel.jsx

@@ -215,7 +215,7 @@ export default function Panel({ onManeuverClick }) {
       </div>
 
       {sheetState !== 'collapsed' && (
-        <div className="px-4 pb-4 overflow-y-auto h-[calc(100%-2rem)]">
+        <div className="px-4 pb-4 overflow-y-auto overflow-x-hidden h-[calc(100%-2rem)]" style={{ paddingBottom: 'max(1rem, env(safe-area-inset-bottom))' }}>
           {header}
           {content}
         </div>