Add Meeting system subject and notification system

- Auto-create a locked "Meeting" subject for every user on registration
  and seed it for all existing users on startup
- Meeting subject cannot be deleted or renamed (is_system flag)
- 5-minute corner toast warning on Dashboard and TV with live countdown,
  dismiss button, and 1-minute re-notify if dismissed
- At start time: full-screen TV overlay with 30-second auto-dismiss,
  automatic pause of running block, switch to Meeting block, and
  auto-start of Meeting timer
- Web Audio API chimes: rising on warnings, falling at meeting start
- Update README with Meeting subject and notification system docs

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

README.md

@@ -20,6 +20,12 @@ A self-hosted web app for managing homeschool schedules, tracking daily learning
 - **Multi-Child Support** — Manage multiple students under one parent account, each with their own color, schedule, and history.
 - **JWT Authentication** — Secure parent login with access tokens and httpOnly refresh cookies. TV dashboard is public (no login required).
 - **Super Admin Panel** — A separate admin interface (at `/super-admin`) for site-wide management. Log in with a dedicated admin username and password (set in `.env`). Lists all registered parent accounts and allows impersonating any user — switching into their session to view and manage their data. An impersonation banner is shown at the top of the screen with a one-click "Exit to Admin Panel" button.
+- **Meeting Subject** — A system subject called "Meeting" (📅) is automatically created for every user and cannot be deleted or renamed. Add it to any schedule block like a normal subject and assign activity options (e.g. agenda items) that will display on the TV during the meeting.
+- **Meeting Notifications** — When a schedule block assigned to the Meeting subject is approaching, the app automatically alerts everyone:
+  - **5-minute warning** — An amber corner toast appears on both the parent Dashboard and the TV with the meeting name and a live countdown. Tap ✕ to dismiss.
+  - **1-minute re-notify** — If the 5-minute toast was dismissed, it reappears at the 1-minute mark.
+  - **At start time** — A full-screen overlay fires on the TV with the meeting name and a 30-second auto-dismiss countdown (tap anywhere to dismiss early). Simultaneously, the currently running block timer is paused, the schedule switches to the Meeting block, and its timer starts automatically. The TV center panel switches to show the meeting's activity options.
+  - **Chime sounds** — A rising three-note chime plays on warnings; a falling chime plays at meeting start. Generated via the Web Audio API — no audio files required.
 
 ---
 
@@ -86,7 +92,8 @@ homeschool/
     └── src/
         ├── composables/
         │   ├── useApi.js         # Axios with auto token-refresh
-        │   └── useWebSocket.js   # Auto-reconnecting WebSocket
+        │   ├── useWebSocket.js   # Auto-reconnecting WebSocket
+        │   └── useMeetingAlerts.js # Meeting countdown, corner toasts, TV overlay, chimes
         ├── stores/               # Pinia: auth, children, schedule, superAdmin
         ├── views/                # LoginView, TVView, DashboardView, LogView, AdminView,
         │                         #   SuperAdminLoginView, SuperAdminView
@@ -153,7 +160,7 @@ Open **http://localhost:8054/login** and register. This creates your admin accou
 ### 5. Set up your data (in order)
 
 1. **Admin** (`/admin`) → Add each child, pick a color
-2. **Admin** → Add subjects (Math, Reading, Science, etc.) with emoji icons and colors. Add activity options to each subject — they appear on the TV dashboard during that block.
+2. **Admin** → Add subjects (Math, Reading, Science, etc.) with emoji icons and colors. Add activity options to each subject — they appear on the TV dashboard during that block. The **Meeting** subject is created automatically and cannot be deleted or renamed, but you can add activity options (agenda items) to it.
 3. **Admin** → Add **Morning Routine** items — these show on the TV during the greeting before the first block starts.
 4. **Admin** → Add **Break Activities** items — these show on the TV center panel whenever a break is active.
 5. **Admin** → Scroll to **Settings** and select your local timezone
@@ -209,7 +216,7 @@ While a session is active, clicking a block in the schedule list **selects** it
 
 | URL | Description |
 |-----|-------------|
-| `/tv/:childId` | Full-screen display — greeting + morning routine, current block timer with subject activities, break timer with break activities, day progress bar (🟢 Start → Finish 🏁), schedule sidebar |
+| `/tv/:childId` | Full-screen display — greeting + morning routine, current block timer with subject activities, break timer with break activities, day progress bar (🟢 Start → Finish 🏁), schedule sidebar, meeting warning toasts, meeting start overlay |
 
 Point a browser on the living room TV at `http://your-lan-ip:8054/tv/1`. The page connects via WebSocket and updates automatically when a parent starts/stops/advances the timer from the Dashboard.