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.

README.md

@@ -490,6 +490,8 @@ Browse the UICC filesystem in a tree view. Files are shown with names, FIDs, and
 - **Read** — reads the selected file (auto-detects transparent vs record files)
 - **Edit** — switch to edit mode, modify hex data, click **Save** to write back
 - **Raw / Decoded** — toggle between hex dump and pysim-decoded JSON view
+- Missing files are shown in red (✗); a present but empty DF shows `(empty)`
+- **Probe all files** — walks the whole tree (incl. custom files), marks every entry present/absent with *N / total* progress, stoppable, and ends with a summary; browsing itself stays lazy
 
 ### Command Hints