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/fs_load.test.js

@@ -0,0 +1,96 @@
+const { test } = require('node:test');
+const assert = require('node:assert');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const html = fs.readFileSync(path.join(__dirname, '..', 'index.html'), 'utf8');
+
+function extractFunc(src, name) {
+	const re = new RegExp('(?:async\\s+)?function\\s+' + name + '\\s*\\([^)]*\\)\\s*\\{');
+	const m = re.exec(src);
+	if (!m) throw new Error('function ' + name + ' not found');
+	let i = m.index + m[0].length - 1;
+	let depth = 0;
+	for (; i < src.length; i++) {
+		if (src[i] === '{') depth++;
+		else if (src[i] === '}') {
+			depth--;
+			if (depth === 0) break;
+		}
+	}
+	return src.slice(m.index, i + 1);
+}
+
+let code = '';
+code += extractFunc(html, 'getParentSel') + '\n';
+code += extractFunc(html, 'pysimFsLoadChildren') + '\n';
+code += 'globalThis.pysimCustomInject = () => {};\n';
+eval(code);
+
+let calls = [];
+let responses = [];
+let renders = 0;
+
+function setup() {
+	calls = [];
+	responses = [];
+	renders = 0;
+	globalThis.pysimFetch = async (p, body) => {
+		calls.push({ path: p, body: JSON.parse(JSON.stringify(body)) });
+		const r = responses.shift();
+		if (r instanceof Error) throw r;
+		return JSON.parse(JSON.stringify(r));
+	};
+	globalThis.pysimFsRenderTree = () => { renders++; };
+	const node = { name: 'DF.USIM', fid: '7fff', isDir: true, children: null, exists: null, parent: { name: 'MF', fid: '3f00' } };
+	return node;
+}
+
+test('tree error payload marks the directory as absent', async () => {
+	const node = setup();
+	responses = [
+		{ success: false, error: 'SW ... 6a82', exists: false },
+		{ success: false, error: 'SW ... 6a82', exists: false },
+	];
+	await pysimFsLoadChildren(node);
+	assert.strictEqual(node.exists, false);
+	assert.deepStrictEqual(node.children, []);
+	assert.strictEqual(renders, 1);
+	assert.strictEqual(calls.length, 2);
+	assert.strictEqual(calls[0].body.parent_sel, 'MF');
+});
+
+test('error payload without exists is not treated as an empty listing', async () => {
+	const node = setup();
+	responses = [
+		{ success: false, error: 'boom' },
+		{ success: false, error: 'boom' },
+	];
+	await pysimFsLoadChildren(node);
+	assert.strictEqual(node.exists, false);
+	assert.deepStrictEqual(node.children, []);
+	assert.strictEqual(renders, 1);
+	assert.strictEqual(calls.length, 1);
+});
+
+test('retry without parent_sel succeeds and maps children', async () => {
+	const node = setup();
+	responses = [
+		{ exists: false },
+		{ exists: true, children: [{ name: 'EF.IMSI', fid: '6f07', isDir: false }] },
+	];
+	await pysimFsLoadChildren(node);
+	assert.strictEqual(node.exists, true);
+	assert.strictEqual(node.children.length, 1);
+	assert.strictEqual(node.children[0].parent, node);
+	assert.strictEqual(node.children[0].exists, true);
+	assert.strictEqual(calls[1].body.parent_sel, undefined);
+});
+
+test('empty successful listing keeps the directory present', async () => {
+	const node = setup();
+	responses = [{ exists: true, children: [] }];
+	await pysimFsLoadChildren(node);
+	assert.strictEqual(node.exists, true);
+	assert.deepStrictEqual(node.children, []);
+});