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.

frontend/tests/html.test.js

@@ -61,3 +61,10 @@ test('custom files form has add/save and cancel controls', () => {
     assert.ok(html.includes("event.key==='Enter')pysimCustomSubmit()"));
     assert.ok(!html.includes('pysimCustomAdd'));
 });
+
+test('file manager has a probe-all-files button and status line', () => {
+    assert.match(html, /id="pysim-fs-probe-btn"[^>]*data-needs="card"/);
+    assert.match(html, /id="pysim-fs-probe-btn"[^>]*data-l10n="Probe all files"/);
+    assert.ok(html.includes('onclick="pysimFsProbeAll()"'));
+    assert.ok(html.includes('id="pysim-fs-probe-status"'));
+});