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/features/room/RoomInput.tsx

@@ -602,6 +602,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
       resetEditor(editor);
       resetEditorHistory(editor);
       setCharCount(0);
+      setMsgDraft([]);
       localStorage.removeItem(`draft-msg-${draftKey}`);
       setReplyDraft(undefined);
       sendTypingStatus(false);
@@ -614,6 +615,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
       replyDraft,
       sendTypingStatus,
       setReplyDraft,
+      setMsgDraft,
       isMarkdown,
       commands,
       setToast,
@@ -687,11 +689,20 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
         });
         resetEditor(editor);
         resetEditorHistory(editor);
+        setMsgDraft([]);
         localStorage.removeItem(`draft-msg-${draftKey}`);
         setReplyDraft(undefined);
         sendTypingStatus(false);
       },
-      [setScheduledMessages, roomId, draftKey, editor, setReplyDraft, sendTypingStatus],
+      [
+        setScheduledMessages,
+        roomId,
+        draftKey,
+        editor,
+        setReplyDraft,
+        setMsgDraft,
+        sendTypingStatus,
+      ],
     );
 
     const handleKeyDown: KeyboardEventHandler = useCallback(