feat(frontend): send initiator-online after sender rejoins with cached/long room ID

- Add forceInitiatorOnline flag to webrtcService.joinRoom
  - Trigger initiator-online for sender when joining with >=8-char IDs

frontend/hooks/useRoomManager.ts

@@ -86,8 +86,17 @@ export function useRoomManager({
             ? shareRoomId
             : roomId;
 
+        // If sender uses a long ID (e.g., cached UUID), proactively send
+        // "initiator-online" after join to trigger receivers' re-handshake.
+        const forceInitiatorOnline =
+          isSenderSide && typeof actualRoomId === "string" && actualRoomId.length >= 8;
+
         // Directly call the service method without dependency injection
-        await webrtcService.joinRoom(actualRoomId, isSenderSide);
+        await webrtcService.joinRoom(
+          actualRoomId,
+          isSenderSide,
+          forceInitiatorOnline
+        );
 
         putMessageInMs(
           messages.text.ClipboardApp.joinRoom.successMsg,

frontend/lib/webrtcService.ts

@@ -141,7 +141,11 @@ class WebRTCService {
   }
 
   // Business methods
-  public async joinRoom(roomId: string, isSender: boolean): Promise<void> {
+  public async joinRoom(
+    roomId: string,
+    isSender: boolean,
+    forceInitiatorOnline: boolean = false
+  ): Promise<void> {
     // Ensure clean state before joining
     if (!isSender) {
       // Force reset FileReceiver state to prevent "already in progress" errors
@@ -149,7 +153,10 @@ class WebRTCService {
     }
 
     const peer = isSender ? this.sender : this.receiver;
-    await peer.joinRoom(roomId, isSender);
+    // If sender is performing a manual reconnect with a cached/long ID,
+    // optionally send an "initiator-online" after successful join so receivers
+    // can reply with "recipient-ready" to re-establish P2P.
+    await peer.joinRoom(roomId, isSender, isSender && !!forceInitiatorOnline);
 
     const setInRoom = isSender
       ? useFileTransferStore.getState().setIsSenderInRoom