feat(nav): draft indicator on room-nav items

Composer drafts persist per room, but nothing in the room list showed
which OTHER rooms had an unsent draft. Add a subtle chat-bubble icon on a
room's nav item when it has a message draft (and isn't the open room), so
half-written messages elsewhere are visible at a glance.

- Shared pure helper hasMsgDraft (utils/draft.ts, unit-tested) replaces
  the inline emptiness check; the composer DraftIndicator now reuses it.
- RoomNavItem reads a memoized selectAtom(draftAtom, hasMsgDraft) so a row
  re-renders only when its draft flag flips (the draft atom is written on
  room-leave, not per keystroke). Uses Icons.Message (pencil is reserved
  for the custom-name marker), muted, aria-label "Unsent draft".
- useHydrateMsgDrafts (mounted in ClientNonUIFeatures) pre-fills the
  per-room draft atoms from draft-msg-* localStorage on startup, so
  indicators are correct after a page reload, not only after revisiting a
  room. Thread drafts (key contains ::) are skipped; room-level only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/DraftIndicator.tsx

@@ -3,7 +3,7 @@ import { useAtomValue } from 'jotai';
 import { Box, Text, config } from 'folds';
 
 import { roomIdToMsgDraftAtomFamily } from '../../state/room/roomInputDrafts';
-import { toPlainText } from '../../components/editor';
+import { hasMsgDraft } from '../../utils/draft';
 import { DraftDot, DraftDotPulse, DraftIndicatorBase } from './DraftIndicator.css';
 
 const PULSE_DURATION = 600;
@@ -27,7 +27,7 @@ type DraftIndicatorProps = {
 export function DraftIndicator({ roomId }: DraftIndicatorProps) {
   const draft = useAtomValue(roomIdToMsgDraftAtomFamily(roomId));
   // Real content, not just an empty paragraph.
-  const hasDraft = toPlainText(draft, false).trim().length > 0;
+  const hasDraft = hasMsgDraft(draft);
 
   const [pulse, setPulse] = useState(false);
   const hadDraft = useRef(false);