feat(anime): add language filter to extensions available list

- Add a language chip row above the Available list; pick one or more languages to narrow the repo index, or "All" to clear
- Show language display names (e.g. "Japanese") instead of raw tags, and an "N of M" count on the Available heading
- Add language-filter.ts with collectLanguages/toggleLanguage/pruneSelection/filterByLanguage/languageLabel, plus tests

src/animeui/language-filter.test.ts

@@ -0,0 +1,49 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  collectLanguages,
+  filterByLanguage,
+  languageLabel,
+  pruneSelection,
+  toggleLanguage,
+} from './language-filter';
+
+const ext = (lang: string, name = lang) => ({ lang, name });
+
+test('languageLabel names known tags and falls back to the raw code', () => {
+  assert.equal(languageLabel('ja'), 'Japanese');
+  assert.equal(languageLabel('all'), 'Multi-language');
+  assert.equal(languageLabel('zzzz'), 'ZZZZ');
+});
+
+test('collectLanguages dedupes, puts multi-language first, then sorts by name', () => {
+  // English, German, Japanese — display names, not codes.
+  assert.deepEqual(collectLanguages([ext('ja'), ext('de'), ext('ja'), ext('all'), ext('en')]), [
+    'all',
+    'en',
+    'de',
+    'ja',
+  ]);
+});
+
+test('toggleLanguage adds, removes, and empties back to All', () => {
+  const one = toggleLanguage(new Set(), 'ja');
+  assert.deepEqual([...one], ['ja']);
+  const two = toggleLanguage(one, 'en');
+  assert.deepEqual([...two].sort(), ['en', 'ja']);
+  assert.deepEqual([...toggleLanguage(two, 'ja')], ['en']);
+  assert.equal(toggleLanguage(one, 'ja').size, 0);
+});
+
+test('pruneSelection drops languages no repository offers any more', () => {
+  assert.deepEqual([...pruneSelection(new Set(['ja', 'de']), ['ja', 'en'])], ['ja']);
+});
+
+test('filterByLanguage keeps everything when nothing is selected', () => {
+  const extensions = [ext('ja'), ext('en'), ext('all')];
+  assert.deepEqual(filterByLanguage(extensions, new Set()), extensions);
+  assert.deepEqual(
+    filterByLanguage(extensions, new Set(['ja', 'all'])).map((extension) => extension.name),
+    ['ja', 'all'],
+  );
+});