fix(nav): harden draft indicator after review

Address findings from 2 review agents on the room-nav draft indicator:

- Icon semantics (a11y): Icons.Message read as message activity and
  collided with two existing bubble uses in the same row (call-chat
  toggle, mark-unread), especially next to the unread badge. Replace it
  with the composer's shared DraftDot (a small color.Success.Main dot),
  so both draft surfaces share one visual language. Rendered as a
  role="img" span with aria-label "Unsent draft" (reliably announced,
  unlike a bare aria-labelled svg).

- Precise thread-key filter: hydration skipped any draftKey containing
  '::', which would also skip an IPv6-literal server name in a roomId.
  Match '::$' (thread root is an event id) so only real thread drafts are
  skipped.

- Defensive hasMsgDraft: guard toPlainText so a corrupted/foreign draft
  value can't throw during a nav render.

- Clear the draft atom on send: the send / scheduled-send handlers reset
  the editor and localStorage but left the jotai draft atom set, so the
  composer DraftIndicator could show a stale dot after sending a restored
  draft. Add setMsgDraft([]) to both.

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

src/app/utils/draft.ts

@@ -10,5 +10,10 @@ export const DRAFT_MSG_KEY_PREFIX = 'draft-msg-';
  * by the composer's DraftIndicator and the room-nav draft indicator.
  */
 export function hasMsgDraft(draft: Descendant[]): boolean {
-  return toPlainText(draft, false).trim().length > 0;
+  try {
+    return toPlainText(draft, false).trim().length > 0;
+  } catch {
+    // A corrupted/foreign draft shape must never crash a render that calls this.
+    return false;
+  }
 }