feat(chat): unify session gateway with stable IDs and a single WS protocol

The frontend previously juggled placeholder IDs, provider-native IDs, and session_created handoffs, which caused race conditions and provider-specific branching. This introduces app-allocated session IDs, a chat run registry with event replay, delta sidebar updates, and one kind-based websocket contract so the UI can treat every provider the same while JSONL remains the source of truth.

server/modules/projects/services/projects-with-sessions-fetch.service.ts

@@ -189,10 +189,11 @@ function readProjectSessionsPageByPath(
   };
 }
 
-// Broadcast progress to all connected WebSocket clients
+// Broadcast progress to all connected WebSocket clients.
+// Uses the unified `kind` envelope like every other websocket frame.
 function broadcastProgress(progress: ProgressUpdate) {
   const message = JSON.stringify({
-    type: 'loading_progress',
+    kind: 'loading_progress',
     ...progress,
   });