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/tests/fs_render.test.js

@@ -56,3 +56,12 @@ test('collapsed directory hides its children', () => {
 	const out = pysimFsRenderNode(node, 0);
 	assert.ok(!out.includes('EF.IMSI'), out);
 });
+
+test('non-existing directory hides previously loaded children', () => {
+	const node = df('DF.WLAN', '5f40', { exists: false, expanded: true, children: [ef('EF.UPLMNWLAN', '4f42')] });
+	const out = pysimFsRenderNode(node, 0);
+	assert.ok(out.includes('✗'), out);
+	assert.ok(!out.includes('EF.UPLMNWLAN'), out);
+	assert.ok(!out.includes('(empty)'), out);
+	assert.ok(!out.includes('pysimFsToggleDir'), out);
+});