feat: add WebSocket client with fallback

- Add Socket.IO client library (v4.7.5)
- Create RFMapperWS class with:
  - Automatic reconnection (5 attempts)
  - HTTP polling fallback
  - Event listener system for scanUpdate, connected, disconnected
  - Floor subscription support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/rf_mapper/web/static/js/websocket.js

@@ -0,0 +1,101 @@
+/**
+ * RF Mapper WebSocket client with automatic reconnection and HTTP fallback
+ */
+class RFMapperWS {
+    constructor() {
+        this.socket = null;
+        this.connected = false;
+        this.reconnectAttempts = 0;
+        this.maxReconnectAttempts = 5;
+        this.listeners = {
+            scanUpdate: [],
+            connected: [],
+            disconnected: []
+        };
+    }
+
+    connect() {
+        // Check if socket.io is loaded
+        if (typeof io === 'undefined') {
+            console.warn('[WS] socket.io not loaded, using HTTP polling');
+            return false;
+        }
+
+        try {
+            this.socket = io('/ws/scan', {
+                transports: ['websocket', 'polling'],
+                reconnection: true,
+                reconnectionDelay: 1000,
+                reconnectionDelayMax: 5000,
+                reconnectionAttempts: this.maxReconnectAttempts
+            });
+
+            this.socket.on('connect', () => {
+                console.log('[WS] Connected');
+                this.connected = true;
+                this.reconnectAttempts = 0;
+                this._emit('connected');
+            });
+
+            this.socket.on('disconnect', (reason) => {
+                console.log('[WS] Disconnected:', reason);
+                this.connected = false;
+                this._emit('disconnected', { reason });
+            });
+
+            this.socket.on('scan_update', (data) => {
+                this._emit('scanUpdate', data);
+            });
+
+            this.socket.on('connect_error', (error) => {
+                console.warn('[WS] Connection error:', error.message);
+                this.reconnectAttempts++;
+                if (this.reconnectAttempts >= this.maxReconnectAttempts) {
+                    console.log('[WS] Max reconnect attempts, falling back to HTTP');
+                    this.connected = false;
+                    this._emit('disconnected', { reason: 'max_reconnect' });
+                }
+            });
+
+            return true;
+        } catch (e) {
+            console.error('[WS] Failed to initialize:', e);
+            return false;
+        }
+    }
+
+    disconnect() {
+        if (this.socket) {
+            this.socket.disconnect();
+            this.socket = null;
+        }
+        this.connected = false;
+    }
+
+    subscribeFloor(floor) {
+        if (this.socket?.connected) {
+            this.socket.emit('subscribe_floor', { floor });
+        }
+    }
+
+    on(event, callback) {
+        if (this.listeners[event]) {
+            this.listeners[event].push(callback);
+        }
+    }
+
+    off(event, callback) {
+        if (this.listeners[event]) {
+            this.listeners[event] = this.listeners[event].filter(cb => cb !== callback);
+        }
+    }
+
+    _emit(event, data) {
+        if (this.listeners[event]) {
+            this.listeners[event].forEach(cb => cb(data));
+        }
+    }
+}
+
+// Global instance
+const rfMapperWS = new RFMapperWS();