feat(web): add WebSocket client and context infrastructure

- WsClient class: connect/disconnect/send/on/off/isConnected/clearCallbacks
- connect() derives wss:// from https:// automatically, returns Promise<void>
- on/off typed with Extract<WsServerMessage, { type: T }> for precise
  callback narrowing, callbacks stored as Map<string, Set<fn>>
- ws-context.ts: WsContextValue type + WsContext definition
- ws-provider.tsx: WsProvider with module-level wsClient singleton,
  owns connection lifecycle (connect/disconnect/isConnected state)
- ws-hooks.ts: useWsClient, useWsConnected, useWsConnect, useWsDisconnect

apps/web/src/lib/ws-client.ts

@@ -12,7 +12,16 @@ export class WsClient {
   private ws: WebSocket | null = null;
   private callbacks = new Map<string, Set<(msg: WsServerMessage) => void>>();
 
+  /**
+   * Called when the WebSocket connection closes.
+   * Set by WsProvider — do not set directly in components.
+   */
   public onError: ((event: Event) => void) | null = null;
+
+  /**
+   * Called when the WebSocket connection encounters an error.
+   * Set by WsProvider — do not set directly in components.
+   */
   public onClose: ((event: CloseEvent) => void) | null = null;
 
   connect(apiUrl: string): Promise<void> {