Fix alert dedup across reloads, add radar fullscreen, fix swipe conflicts

- Persist alertIdsSeen to localStorage so already-seen alerts don't
  re-notify every time the PWA is opened; cap stored IDs at 200 entries
- Use serviceWorker.showNotification() for alert notifications so they
  work when the app is backgrounded, with fallback to new Notification()
- Add ⛶ fullscreen button to radar embed; uses Fullscreen API and
  resizes the MapLibre canvas after toggle
- Fix swipe-to-change-tab: block gesture when touch starts on a map,
  hourly scroll row, or radar timeline; also require the gesture to be
  more horizontal than vertical (dx > dy*1.5) to avoid conflict with
  vertical scrolling

app/public/index.html

@@ -194,13 +194,14 @@
           <button class="link-btn" id="open-map-tab">Open full map →</button>
         </span>
       </div>
-      <div class="radar-embed-wrap">
+      <div class="radar-embed-wrap" id="radar-embed-wrap">
         <div id="radar-embed" class="radar-map"></div>
         <div class="radar-controls">
           <button id="embed-play" class="radar-btn play-btn" title="Play/Pause">▶</button>
           <button id="embed-layer" class="radar-btn layer-btn" title="Toggle radar/satellite">R</button>
           <div class="radar-timeline" id="embed-timeline"></div>
           <span id="embed-timestamp" class="radar-ts">--</span>
+          <button id="embed-fullscreen" class="radar-btn fullscreen-btn" title="Toggle fullscreen">⛶</button>
         </div>
       </div>