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/MessageList.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef, useState } from 'react'
+import { useEffect, useMemo, useRef, useState } from 'react'
 import { getRoomFileUrl, getRoomImageUrl } from '../api/rooms'
 import { useAuth } from '../context/AuthContext'
 import { formatFileSize } from '../lib/fileSize'
@@ -76,6 +76,7 @@ export function MessageList({ roomId, messages, members, onEdit, onReact }: Mess
   const [reactingId, setReactingId] = useState<string | null>(null)
   const [reactionPlacement, setReactionPlacement] = useState<'above' | 'below'>('below')
   const [previewFile, setPreviewFile] = useState<MessageFileInfo | null>(null)
+  const memberUsernames = useMemo(() => new Set(members.map((m) => m.username)), [members])
 
   function displayNameForUserId(userId: string): string {
     const member = members.find((m) => m.user_id === userId)
@@ -166,7 +167,7 @@ export function MessageList({ roomId, messages, members, onEdit, onReact }: Mess
                   )}
                   {msg.content && (
                     <div className="message-text">
-                      <MessageContent content={msg.content} />
+                      <MessageContent content={msg.content} memberUsernames={memberUsernames} />
                       {msg.edited_at && <span className="message-edited"> (edited)</span>}
                     </div>
                   )}