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/api/chats.ts

@@ -150,6 +150,14 @@ export async function deleteChat(chatId: number, forAll: boolean): Promise<void>
   await http.delete(`/chats/${chatId}`, { params: { for_all: forAll } });
 }
 
+export async function clearChat(chatId: number): Promise<void> {
+  await http.post(`/chats/${chatId}/clear`);
+}
+
+export async function deleteMessage(messageId: number, forAll = false): Promise<void> {
+  await http.delete(`/messages/${messageId}`, { params: { for_all: forAll } });
+}
+
 export async function discoverChats(query?: string): Promise<DiscoverChat[]> {
   const { data } = await http.get<DiscoverChat[]>("/chats/discover", {
     params: query?.trim() ? { query: query.trim() } : undefined