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/Composer.css

@@ -13,9 +13,14 @@
   align-items: flex-end;
 }
 
-.composer-box textarea {
+.composer-textarea-wrap {
+  position: relative;
   flex: 1;
   min-width: 0;
+}
+
+.composer-box textarea {
+  width: 100%;
   resize: none;
   background: var(--ds-surface-2);
   border: 1px solid var(--ds-border);