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_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);