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/hooks/useHydrateMsgDrafts.ts

@@ -0,0 +1,43 @@
+import { useEffect } from 'react';
+import { useStore } from 'jotai';
+import { Descendant } from 'slate';
+import { roomIdToMsgDraftAtomFamily } from '../state/room/roomInputDrafts';
+import { DRAFT_MSG_KEY_PREFIX, hasMsgDraft } from '../utils/draft';
+
+/**
+ * On startup, pre-fill the per-room message-draft atoms from their localStorage
+ * persistence (`draft-msg-<roomId>`). RoomInput only restores a draft into its
+ * atom when that room's composer mounts, so without this a background room's
+ * persisted draft wouldn't reflect in the room-nav draft indicator until the
+ * room is opened. Runs once; only touches the same atoms RoomInput restores from
+ * (identical content), so composer restore is unaffected.
+ *
+ * Thread drafts (key contains `::`) are skipped — the nav indicator is room-level.
+ */
+export function useHydrateMsgDrafts(): void {
+  const store = useStore();
+
+  useEffect(() => {
+    let keys: string[];
+    try {
+      keys = Object.keys(localStorage);
+    } catch {
+      return;
+    }
+    keys.forEach((key) => {
+      if (!key.startsWith(DRAFT_MSG_KEY_PREFIX)) return;
+      const draftKey = key.slice(DRAFT_MSG_KEY_PREFIX.length);
+      if (!draftKey || draftKey.includes('::')) return; // room-level drafts only
+      try {
+        const stored = localStorage.getItem(key);
+        if (!stored) return;
+        const nodes = JSON.parse(stored) as Descendant[];
+        if (Array.isArray(nodes) && hasMsgDraft(nodes)) {
+          store.set(roomIdToMsgDraftAtomFamily(draftKey), nodes);
+        }
+      } catch {
+        // Ignore a malformed stored draft.
+      }
+    });
+  }, [store]);
+}