feat(phase7): per-channel notification settings and read receipts

Phase 7.1 — Per-channel notification settings
- New notification_settings table (user_id, channel_id, level)
- GET/PUT/DELETE handlers under /channels/{channelID}/notifications
- Push dispatch and @mention loops filtered by notification level
- Frontend: bell icon per channel cycling all/mentions/none

Phase 7.4 — Read receipts
- New read_states table (user_id, channel_id, last_read_message_id)
- PUT /channels/{channelID}/read and GET /users/me/read-states
- Auto-mark-read when messages load in ChatArea
- Unread dot for channels never opened

Also: favicon/icon refresh in index.html

web/src/stores/notificationSettings.ts

@@ -0,0 +1,34 @@
+import { create } from 'zustand';
+import { api } from '../lib/api';
+
+type NotificationLevel = 'all' | 'mentions' | 'none';
+
+interface NotificationSettingsState {
+  settings: Record<string, NotificationLevel>;
+  initialized: boolean;
+  loaded: boolean;
+  fetchSettings: () => Promise<void>;
+  setLevel: (channelId: string, level: NotificationLevel) => Promise<void>;
+}
+
+export const useNotificationSettingsStore = create<NotificationSettingsState>()((set) => ({
+  settings: {},
+  initialized: false,
+  loaded: false,
+
+  fetchSettings: async () => {
+    try {
+      const settings = await api.get<Record<string, NotificationLevel>>('/channels/me/notifications');
+      set({ settings, loaded: true, initialized: true });
+    } catch {
+      set({ loaded: true, initialized: true });
+    }
+  },
+
+  setLevel: async (channelId: string, level: NotificationLevel) => {
+    await api.put(`/channels/${channelId}/notifications`, { level });
+    set((state) => ({
+      settings: { ...state.settings, [channelId]: level },
+    }));
+  },
+}));