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>

backend/app/routers/auth.py

@@ -11,6 +11,7 @@ from app.auth.jwt import (
 )
 from app.dependencies import get_db
 from app.models.user import User
+from app.models.subject import Subject
 from app.schemas.auth import LoginRequest, RegisterRequest, TokenResponse
 from app.schemas.user import UserOut
 
@@ -39,6 +40,16 @@ async def register(body: RegisterRequest, response: Response, db: AsyncSession =
     await db.commit()
     await db.refresh(user)
 
+    meeting = Subject(
+        user_id=user.id,
+        name="Meeting",
+        icon="📅",
+        color="#6366f1",
+        is_system=True,
+    )
+    db.add(meeting)
+    await db.commit()
+
     access = create_access_token({"sub": str(user.id)})
     refresh = create_refresh_token({"sub": str(user.id)})
     response.set_cookie(REFRESH_COOKIE, refresh, **COOKIE_OPTS)