fix: drop stale STK dialogs, finish pending on TP re-send, mute client-gone (v3.6.35)

The live log showed the "stuck" state: after the menu watchdog (60 s)
answered a pending command, the PWA kept the panel; the next click got
{'error': 'no pending command'}, 400 and that reply was fed to the dialog
handler as if it were a card answer.  A page reload then cancelled in-flight
polls, flooding the log with BrokenPipeError tracebacks.

- stkMenuRespond detects the already-answered reply (stkNoPendingError) and
  closes the panel (stkMenuPanelReset) instead of treating it as an answer;
  the 5 s status poll drops the panel whenever the server reports
  pending: false, even when the Phone tab is hidden.
- /api/terminal-profile (POST) and /api/rescue finish a pending command
  with a cancel TR first, so re-sending the TP can never shadow an
  unanswered FETCH.
- _send_json and _serve_static swallow BrokenPipe/ConnectionReset/
  ConnectionAborted with one "CLIENT GONE" line (a reload cancels polls);
  the handlers' state changes still stand.

Tests: stkNoPendingError + the already-answered respond path; version
3.6.35; sw simple-v308; stylesheet rebuilt.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v307';
+const CACHE = 'simple-v308';
 const URLS = [
   'index.html',
   'help.html',