ui: hide, don't clear, children of non-selectable DFs

- pysimFsLoadChildren failure now only marks exists=false and re-renders;
  loaded children stay in the node and are hidden by the renderer (red
  cross, no toggle, no (empty)), so a failed DF is re-attempted on the
  next probe/refresh instead of being short-circuited
- dropped the parent_sel-less /api/tree retry: error payloads now carry
  exists:false, so it doubled requests for every absent DF and re-selected
  the same FID without its parent (pySim probe_file fallback)
- probe walks strictly on exists===true and never collects files under a
  non-existing DF; no longer clears children on a failed EF select
- tests updated: single request per failed load, children untouched,
  200 exists:false marks absent without retry, render hides stale
  children, probe never fetches/selects them; SW cache v116 -> v117.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v116';
+const CACHE = 'otaman-v117';
 const URLS = [
   'index.html',
   'help.html',