Add @-mention highlighting, sidebar badge, and push customization (#39)

@username tokens in a sent message are parsed against the room's actual
members (skipping fenced/inline code, so pasted code isn't misread) and
recorded as MessageMention rows, reusing #38's read-tracking and
offline-member broadcast infrastructure rather than building a parallel
notification path:

- Sidebar: a mentioned-and-unread room shows a distinct highlight-
  colored badge instead of (not alongside) the plain unread dot --
  computed the same way as has_unread, just scoped to messages that
  mention the caller, and cleared by the same last_read_at mark-read
  flow.
- Push notifications: a mentioned offline recipient gets "X mentioned
  you: ..." instead of the generic "X: ...", still per-recipient since
  the same message can page some room members and not others.
- Message rendering: a validated @username is highlighted inline,
  implemented by turning it into a `[@username](mention:username)` link
  before markdown parsing and overriding link rendering to style
  `mention:`-scheme links as a span instead of an anchor -- reuses
  markdown-to-jsx's existing parser rather than hand-rolling text-node
  splitting.
- Composer: typing @ opens an autocomplete dropdown of matching room
  members (arrow keys to navigate, Enter/Tab/click to insert, Escape or
  moving the cursor away to dismiss).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/services/room_service.py

@@ -5,7 +5,7 @@ from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
-from app.models import Message, Room, RoomMembership, RoomRole, User
+from app.models import Message, MessageMention, Room, RoomMembership, RoomRole, User
 from app.schemas.room import RoomCreate, RoomUpdate
 from app.services.email_service import send_email
 
@@ -81,22 +81,38 @@ async def list_open_rooms(db: AsyncSession, user_id: uuid.UUID) -> list[tuple[Ro
 
 async def list_member_rooms(
     db: AsyncSession, user_id: uuid.UUID
-) -> list[tuple[Room, RoomRole, bool]]:
+) -> list[tuple[Room, RoomRole, bool, bool]]:
     last_message_at = (
         select(func.max(Message.created_at))
         .where(Message.room_id == Room.id)
         .correlate(Room)
         .scalar_subquery()
     )
+    # Unread AND mentions this user specifically -- a stronger signal than
+    # plain has_unread, surfaced as its own field so the sidebar can show a
+    # visually distinct badge instead of (not alongside) the plain dot.
+    has_unread_mention = (
+        select(MessageMention.message_id)
+        .join(Message, Message.id == MessageMention.message_id)
+        .where(
+            MessageMention.user_id == user_id,
+            Message.room_id == Room.id,
+            Message.created_at > RoomMembership.last_read_at,
+        )
+        .correlate(Room, RoomMembership)
+        .exists()
+    )
     result = await db.execute(
-        select(Room, RoomMembership.role, RoomMembership.last_read_at, last_message_at)
+        select(
+            Room, RoomMembership.role, RoomMembership.last_read_at, last_message_at, has_unread_mention
+        )
         .join(RoomMembership, RoomMembership.room_id == Room.id)
         .where(RoomMembership.user_id == user_id)
         .order_by(Room.created_at)
     )
     return [
-        (room, role, last_message_at is not None and last_message_at > last_read_at)
-        for room, role, last_read_at, last_message_at in result.all()
+        (room, role, last_message_at is not None and last_message_at > last_read_at, has_mention)
+        for room, role, last_read_at, last_message_at, has_mention in result.all()
     ]