ui: probe-all-files walk and honest file tree coloring

- pysimFsLoadChildren now treats {success:false,error} (and any error
  payload) as a failed listing: absent DFs/ADFs render as a red cross
  without an expand arrow instead of silently opening empty; the
  /api/tree 500 body also carries exists:false for shape consistency
  with /api/select
- an expanded directory with zero children shows a gray (empty)
  placeholder
- new Probe all files button (data-needs=card) in the tree header:
  walks the whole model tree from MF, verifies every DF/ADF via
  /api/tree and every file (incl. custom entries) via /api/select,
  skips subtrees of absent dirs, shows N/total progress, toggles to
  Stop, and reports present/absent counts with elapsed time; results
  colour the current tree only
- tests: fs_load (error/retry/empty), fs_render (red cross, (empty)),
  fs_probe (flags, absent-dir skip, custom files, stop, summary);
  html structural check; help EN/RU, READMEs, AGENTS updated;
  SW cache v115 -> v116.

pysim_otaman_server/server.py

@@ -2184,7 +2184,7 @@ class PysimHandler(BaseHTTPRequestHandler):
                 sys.stderr.write('Handler error: %s\n' % e)
                 if 'Card' in str(e) or 'Transaction' in str(e) or 'Transmit' in str(e):
                     _handle_card_disconnect()
-                err = {'success': False, 'error': str(e)}
+                err = {'success': False, 'error': str(e), 'exists': False}
                 self._send_json(err, 500)
                 self._log_resp(err)
         elif self.path == '/api/menu-select':