feat(core): clear saved chat and add message deletion scopes

backend:

- add message_hidden table for per-user message hiding

- support DELETE /messages/{id}?for_all=true|false

- implement delete-for-me vs delete-for-all logic by chat type/permissions

- add POST /chats/{chat_id}/clear and route saved chat deletion to clear

web:

- saved messages action changed from delete to clear

- message context menu now supports delete modal: for me / for everyone

- add local store helpers removeMessage/clearChatMessages

- include realtime stability improvements and app error boundary

web/src/main.tsx

@@ -1,10 +1,13 @@
 import React from "react";
 import ReactDOM from "react-dom/client";
 import { App } from "./app/App";
+import { AppErrorBoundary } from "./components/AppErrorBoundary";
 import "./index.css";
 
 ReactDOM.createRoot(document.getElementById("root")!).render(
   <React.StrictMode>
-    <App />
+    <AppErrorBoundary>
+      <App />
+    </AppErrorBoundary>
   </React.StrictMode>
 );