Persist stats exclusions in DB and fix word metrics filtering

- Stats vocabulary exclusions stored in `imm_stats_excluded_words` (schema v18); seeded from localStorage on first load
- Session, overview, trends, and library word metrics use filtered persisted occurrences with raw fallback
- Session known-word % chart uses filtered persisted totals as denominator for both known and total
- JLPT subtitle styling changed to underline-only; no longer overrides text color

stats/src/hooks/useExcludedWords.test.ts

@@ -0,0 +1,87 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  getExcludedWordsSnapshot,
+  initializeExcludedWordsStore,
+  resetExcludedWordsStoreForTests,
+  setExcludedWords,
+} from './useExcludedWords';
+import { BASE_URL } from '../lib/api-client';
+
+const STORAGE_KEY = 'subminer-excluded-words';
+
+function installLocalStorage(initial: Record<string, string> = {}) {
+  const values = new Map(Object.entries(initial));
+  Object.defineProperty(globalThis, 'localStorage', {
+    configurable: true,
+    value: {
+      getItem: (key: string) => values.get(key) ?? null,
+      setItem: (key: string, value: string) => values.set(key, value),
+      removeItem: (key: string) => values.delete(key),
+    },
+  });
+  return values;
+}
+
+test('initializeExcludedWordsStore seeds empty database exclusions from localStorage', async () => {
+  resetExcludedWordsStoreForTests();
+  const localRows = [{ headword: '猫', word: '猫', reading: 'ねこ' }];
+  const storage = installLocalStorage({ [STORAGE_KEY]: JSON.stringify(localRows) });
+  const originalFetch = globalThis.fetch;
+  const requests: Array<{ url: string; method: string; body: string }> = [];
+  globalThis.fetch = (async (input: RequestInfo | URL, init?: RequestInit) => {
+    requests.push({
+      url: String(input),
+      method: init?.method ?? 'GET',
+      body: String(init?.body ?? ''),
+    });
+    if (!init?.method) {
+      return new Response(JSON.stringify([]), {
+        status: 200,
+        headers: { 'Content-Type': 'application/json' },
+      });
+    }
+    return new Response(JSON.stringify({ ok: true }), { status: 200 });
+  }) as typeof globalThis.fetch;
+
+  try {
+    await initializeExcludedWordsStore();
+
+    assert.deepEqual(getExcludedWordsSnapshot(), localRows);
+    assert.deepEqual(requests, [
+      { url: `${BASE_URL}/api/stats/excluded-words`, method: 'GET', body: '' },
+      {
+        url: `${BASE_URL}/api/stats/excluded-words`,
+        method: 'PUT',
+        body: JSON.stringify({ words: localRows }),
+      },
+    ]);
+    assert.equal(storage.get(STORAGE_KEY), JSON.stringify(localRows));
+  } finally {
+    globalThis.fetch = originalFetch;
+    resetExcludedWordsStoreForTests();
+  }
+});
+
+test('setExcludedWords updates the database-backed exclusion list', async () => {
+  resetExcludedWordsStoreForTests();
+  const storage = installLocalStorage();
+  const originalFetch = globalThis.fetch;
+  let seenBody = '';
+  globalThis.fetch = (async (_input: RequestInfo | URL, init?: RequestInit) => {
+    seenBody = String(init?.body ?? '');
+    return new Response(JSON.stringify({ ok: true }), { status: 200 });
+  }) as typeof globalThis.fetch;
+
+  try {
+    const rows = [{ headword: 'する', word: 'する', reading: 'する' }];
+    await setExcludedWords(rows);
+
+    assert.deepEqual(getExcludedWordsSnapshot(), rows);
+    assert.equal(seenBody, JSON.stringify({ words: rows }));
+    assert.equal(storage.get(STORAGE_KEY), JSON.stringify(rows));
+  } finally {
+    globalThis.fetch = originalFetch;
+    resetExcludedWordsStoreForTests();
+  }
+});