feat: Add Chat UX improvements with notifications and @mention support

- Add ActionBar component with expandable toolbar for mobile
- Add @mention functionality with autocomplete dropdown
- Add browser notification system (push, sound, vibration)
- Add NotificationSettings modal for user preferences
- Add mention badges on room list cards
- Add ReportPreview with Markdown rendering and copy/download
- Add message copy functionality with hover actions
- Add backend mentions field to messages with Alembic migration
- Add lots field to rooms, remove templates
- Optimize WebSocket database session handling
- Various UX polish (animations, accessibility)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/stores/mentionStore.ts

@@ -0,0 +1,58 @@
+/**
+ * Store for tracking unread @mentions across rooms
+ * Uses localStorage for persistence
+ */
+import { create } from 'zustand'
+import { persist } from 'zustand/middleware'
+
+interface MentionState {
+  // Map of roomId -> unread mention count
+  unreadMentions: Record<string, number>
+
+  // Add a mention for a room
+  addMention: (roomId: string) => void
+
+  // Clear mentions for a room (when user views the room)
+  clearMentions: (roomId: string) => void
+
+  // Get mention count for a room
+  getMentionCount: (roomId: string) => number
+
+  // Get total unread mentions across all rooms
+  getTotalMentions: () => number
+}
+
+export const useMentionStore = create<MentionState>()(
+  persist(
+    (set, get) => ({
+      unreadMentions: {},
+
+      addMention: (roomId: string) => {
+        set((state) => ({
+          unreadMentions: {
+            ...state.unreadMentions,
+            [roomId]: (state.unreadMentions[roomId] || 0) + 1,
+          },
+        }))
+      },
+
+      clearMentions: (roomId: string) => {
+        set((state) => {
+          const { [roomId]: _, ...rest } = state.unreadMentions
+          return { unreadMentions: rest }
+        })
+      },
+
+      getMentionCount: (roomId: string) => {
+        return get().unreadMentions[roomId] || 0
+      },
+
+      getTotalMentions: () => {
+        return Object.values(get().unreadMentions).reduce((sum, count) => sum + count, 0)
+      },
+    }),
+    {
+      name: 'task_reporter_mentions',
+    }
+  )
+)