Add CSS-based fullscreen for Map tab; fix state desync bug

- Add fullscreen toggle button (⛶/✕) to Map tab controls row
- CSS class `map-fs` hides header/nav and expands map to fill screen
- Drop unreliable requestFullscreen() API (blocked on iOS/PWA) in favor of pure CSS approach
- Fix button state desync: syncBtn() runs on init so stale ✕ state can't persist across reloads
- Shrink embedded radar height on mobile (220px) and tune dvh calculations for safe-area insets

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

app/public/index.html

@@ -191,7 +191,7 @@
       <div class="section-head">
         <span class="section-label">Radar</span>
         <span class="section-note">
-          <button class="link-btn" id="open-map-tab">Open full map →</button>
+          <button class="link-btn" id="open-map-tab">Open map →</button>
         </span>
       </div>
       <div class="radar-embed-wrap" id="radar-embed-wrap">
@@ -257,6 +257,7 @@
             <option value="light">Light</option>
             <option value="satellite">Satellite</option>
           </select>
+          <button id="map-fullscreen-btn" class="radar-btn radar-fs-btn" title="Fullscreen">⛶</button>
         </div>
         
         <div class="map-layer-menu">