file manager: scaffold the tree filter (multiselect)

Add a Filter multiselect next to the sort pills in the file tree toolbar.
The option list is large and undecided, so FS_FILTER_OPTIONS is empty and
the per-node match is a no-op for now; the mechanism is wired end to end:

- FS_FILTER_OPTIONS const (empty), pysimFsFilter state persisted to
  simple_fs_filter, pysimFsSetFilter / pysimFsFilterMatch /
  pysimFsFilterRender, and the render loop consults pysimFsFilterMatch.
- The dropdown re-renders on language switch; "Filter:" and
  "(no filters yet)" are translated (EN/RU).

Frontend 1132 tests green (6 new scaffold pins).

frontend/tests/fs_filter.test.js

@@ -0,0 +1,56 @@
+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('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);
+}
+
+// module state the extracted match function closes over
+let pysimFsFilter = new Set();
+eval(extractFunc(html, 'pysimFsFilterMatch'));
+
+test('the file tree has a multiselect filter next to the sort pills', () => {
+	assert.match(html, /id="pysim-fs-filter"[^>]*multiple[^>]*onchange="pysimFsSetFilter\(\)"/);
+	assert.match(html, /data-l10n="Filter:"/);
+});
+
+test('the filter options list is scaffolded and empty for now', () => {
+	assert.match(html, /const FS_FILTER_OPTIONS = \[/);
+});
+
+test('the render consults the filter when walking children', () => {
+	assert.match(html, /pysimFsFilterMatch\(child\)/);
+});
+
+test('the filter state is persisted under simple_fs_filter', () => {
+	const fn = extractFunc(html, 'pysimFsSetFilter');
+	assert.ok(fn.indexOf("localStorage.setItem('simple_fs_filter'") > 0, fn);
+	assert.ok(fn.indexOf('pysimFsRenderTree()') > 0, fn);
+});
+
+test('the dropdown is re-rendered on language switch', () => {
+	const fn = extractFunc(html, 'refreshDynamicI18n');
+	assert.ok(fn.indexOf('pysimFsFilterRender()') > 0, fn);
+});
+
+test('an empty filter matches everything (scaffold no-op until criteria land)', () => {
+	pysimFsFilter = new Set();
+	assert.strictEqual(pysimFsFilterMatch({ name: 'EF.ADN', fid: '6F3A' }), true);
+	assert.strictEqual(pysimFsFilterMatch({ name: 'DF.GSM', fid: '7F20', isDir: true }), true);
+});

frontend/tests/fs_render.test.js

@@ -22,8 +22,10 @@ function extractFunc(src, name) {
 }
 
 let code = 'var pysimFsSort = "fid";\n';
+code += 'var pysimFsFilter = new Set();\n';
 code += extractFunc(html, 'pysimFsSortChildren') + '\n';
 code += extractFunc(html, 'pysimFsNodePath') + '\n';
+code += extractFunc(html, 'pysimFsFilterMatch') + '\n';
 code += extractFunc(html, 'pysimFsRenderNode') + '\n';
 code += 'globalThis.esc = s => s;\nglobalThis.t = s => s;\n';
 eval(code);