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.

src/components/main-content/types/types.ts

@@ -44,7 +44,6 @@ export type MainContentProps = {
   setActiveTab: Dispatch<SetStateAction<AppTab>>;
   ws: WebSocket | null;
   sendMessage: (message: unknown) => void;
-  latestMessage: unknown;
   isMobile: boolean;
   onMenuClick: () => void;
   isLoading: boolean;

src/components/main-content/view/MainContent.tsx

@@ -37,7 +37,6 @@ function MainContent({
   setActiveTab,
   ws,
   sendMessage,
-  latestMessage,
   isMobile,
   onMenuClick,
   isLoading,
@@ -126,7 +125,6 @@ function MainContent({
                 selectedSession={selectedSession}
                 ws={ws}
                 sendMessage={sendMessage}
-                latestMessage={latestMessage}
                 onFileOpen={handleFileOpen}
                 onInputFocusChange={onInputFocusChange}
                 onSessionProcessing={onSessionProcessing}