fix: persist active_remote_id in state for remote session restore

Remote sessions failed to reconnect after page refresh because
restoreState() and renderSheetList() didn't pass remoteId to
openSession(). Fix:

- state.py: add active_remote_id field to empty_state()
- main.py: extend StatePatch model to accept active_session and
  active_remote_id; update PATCH /api/state to selectively update
  only fields explicitly provided (using model_fields_set)
- app.js restoreState(): read state.active_remote_id and pass it
  as remoteId option so page-refresh reconnects remote sessions
- app.js openSession(): fire-and-forget PATCH /api/state to persist
  active_session + active_remote_id after successful connect
- app.js closeSession(): fire-and-forget PATCH /api/state to clear
  both fields so refresh doesn't try to reopen a closed session
- app.js renderSheetList(): add data-remote-id to sheet items and
  pass remoteId in click handler (previously called openSession(name)
  with no remoteId, routing remote sessions to local 404 endpoint)

Fixes: POST /api/sessions/paperclip-adapter/connect 404 when clicking
a remote session from the mobile bottom sheet or after page refresh.

muxplex/state.py

@@ -59,6 +59,7 @@ def empty_state() -> dict:
     """
     return {
         "active_session": None,
+        "active_remote_id": None,
         "session_order": [],
         "sessions": {},
         "devices": {},