fix: read receipts (single/double check), push notifications for all users

- Track when others read messages via messages_read WS event
- Show Check (single) for sent, CheckCheck (double) only when read
- Send push to all recipients, SW suppresses if app is focused

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/store/index.ts

@@ -9,6 +9,7 @@ interface AppStore {
   onlineUsers: Set<string>;
   typingUsers: Record<string, string[]>;
   connected: boolean;
+  chatReadAt: Record<string, string>; // chatId → ISO timestamp others last read
 
   setUser: (user: User | null) => void;
   setChats: (chats: Chat[]) => void;
@@ -23,6 +24,7 @@ interface AppStore {
   setTyping: (chatId: string, userId: string, typing: boolean) => void;
   setConnected: (v: boolean) => void;
   markRead: (chatId: string) => void;
+  markReadByOther: (chatId: string) => void;
   logout: () => void;
 }
 
@@ -41,6 +43,7 @@ export const useStore = create<AppStore>((set, get) => ({
   onlineUsers: new Set(),
   typingUsers: {},
   connected: false,
+  chatReadAt: {},
 
   setUser: (user) => set({ user }),
 
@@ -120,9 +123,13 @@ export const useStore = create<AppStore>((set, get) => ({
     chats: state.chats.map(c => c.id === chatId ? { ...c, unreadCount: 0 } : c)
   })),
 
+  markReadByOther: (chatId) => set(state => ({
+    chatReadAt: { ...state.chatReadAt, [chatId]: new Date().toISOString() }
+  })),
+
   logout: () => {
     localStorage.removeItem('jc_token');
     localStorage.removeItem('jc_user');
-    set({ user: null, chats: [], activeChat: null, messages: {}, connected: false });
+    set({ user: null, chats: [], activeChat: null, messages: {}, connected: false, chatReadAt: {} });
   },
 }));