feat: add socket event contract for Tauri integration

- Documented the socket event contract for communication between the backend and frontend in Tauri mode.
- Introduced constants for `REQUEST_ENCRYPTION_KEY` and `ENCRYPTION_KEY_EVENT` to facilitate encryption key management.
- Updated `setupTauriSocketListeners` to handle the encryption key request and response flow.

docs/src/03-services.md

@@ -170,6 +170,17 @@ const socket = io(BACKEND_URL, {
 });
 ```
 
+### Socket event contract (Tauri)
+
+In Tauri mode, the Rust socket forwards server events to the frontend via the `server:event` Tauri event. The frontend listens and can emit back via `emitViaRustSocket` (see `utils/tauriSocket.ts`).
+
+| Direction   | Event name                 | Payload                         | Description |
+|------------|----------------------------|----------------------------------|-------------|
+| Backend →  | `request:encryption_key`   | (any)                            | Backend requests the client’s encryption key. |
+| Frontend → | `encryption_key`           | `{ encryptionKey: string \| null }` | Frontend sends `encryptionKeyByUser` for the current user from authSlice. |
+
+Constants: `REQUEST_ENCRYPTION_KEY`, `ENCRYPTION_KEY_EVENT` (exported from `utils/tauriSocket.ts`).
+
 ## MTProto Service (`services/mtprotoService.ts`)
 
 Telegram MTProto client singleton.

src/utils/tauriSocket.ts

@@ -12,6 +12,10 @@
  * Legacy bridge (for backwards compatibility during migration):
  * - socket:should_connect / socket:should_disconnect
  * - report_socket_connected / disconnected / error
+ *
+ * Socket event contract (backend <-> frontend):
+ * - Backend emits: REQUEST_ENCRYPTION_KEY — frontend should reply with encryption key.
+ * - Frontend emits: ENCRYPTION_KEY_EVENT — payload { encryptionKey: string | null } from authSlice.
  */
 import { isTauri as coreIsTauri, invoke } from '@tauri-apps/api/core';
 import { listen, UnlistenFn } from '@tauri-apps/api/event';
@@ -20,6 +24,12 @@ import { store } from '../store';
 import { setSocketIdForUser, setStatusForUser } from '../store/socketSlice';
 import { BACKEND_URL } from './config';
 
+/** Event name the backend emits to request the client’s encryption key. */
+export const REQUEST_ENCRYPTION_KEY = 'request:encryption_key';
+
+/** Event name the frontend emits with encryptionKeyByUser (authSlice) for the current user. */
+export const ENCRYPTION_KEY_EVENT = 'encryption_key';
+
 // Check if we're running in Tauri
 export const isTauri = (): boolean => {
   return coreIsTauri();
@@ -154,8 +164,16 @@ export async function setupTauriSocketListeners(): Promise<void> {
 
     // Listen for forwarded server events
     unlistenServerEvent = await listen<{ event: string; data: unknown }>('server:event', event => {
-      console.log('[TauriSocket] Server event:', event.payload.event, event.payload.data);
-      // Future: dispatch to specific handlers based on event type
+      const { event: eventName, data } = event.payload;
+      console.log('[TauriSocket] Server event:', eventName, data);
+
+      if (eventName === REQUEST_ENCRYPTION_KEY) {
+        const state = store.getState();
+        const userId =
+          state.user.user?._id ?? getSocketUserId();
+        const encryptionKey = state.auth.encryptionKeyByUser[userId] ?? null;
+        void emitViaRustSocket(ENCRYPTION_KEY_EVENT, { encryptionKey });
+      }
     });
 
     // Legacy: Listen for connect requests from Rust (backwards compat)