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>

frontend/src/components/RoomRow.tsx

@@ -32,7 +32,12 @@ export function RoomRow({ room, colorIndex, active }: RoomRowProps) {
         </div>
         {room.description && <div className="room-row-subtitle">{room.description}</div>}
       </div>
-      {room.has_unread && !active && <span className="room-row-unread-dot" aria-label="Unread messages" />}
+      {!active && room.has_mention && (
+        <span className="room-row-mention-dot" aria-label="You were mentioned" />
+      )}
+      {!active && !room.has_mention && room.has_unread && (
+        <span className="room-row-unread-dot" aria-label="Unread messages" />
+      )}
     </Link>
   )
 }