Add floating version badge in top-left corner

Small yellow badge fixed at position (4, 4) showing the running build
version. Helps visually confirm which bundle a browser is loading when
diagnosing cache issues.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/map/MapLayout.tsx

@@ -71,6 +71,7 @@ export const MapLayout: React.FC<Props> = ({ data }) => {
           socket={data.socketRef.current} />
         <RareNotification recentRares={data.recentRares} />
         <DeathNotification deathAlerts={data.deathAlerts} />
+        {version && <div className="ml-version-badge">v{version}</div>}
       </div>
     </WindowManagerProvider>
   );

frontend/src/styles/map-layout.css

@@ -389,6 +389,24 @@
   margin-bottom: 2px;
 }
 
+/* Floating version badge in the top-left of the viewport.
+   Lets us verify at a glance which build is running. */
+.ml-version-badge {
+  position: fixed;
+  top: 4px;
+  left: 4px;
+  z-index: 10000;
+  padding: 3px 8px;
+  background: rgba(0, 0, 0, 0.75);
+  border: 1px solid rgba(255, 200, 64, 0.5);
+  border-radius: 4px;
+  color: #ffcc44;
+  font-family: monospace;
+  font-size: 0.7rem;
+  pointer-events: none;
+  letter-spacing: 0.02em;
+}
+
 /* ── Tooltip ──────────────────────────────────────────── */
 .ml-tooltip {
   position: absolute;