fix(stats): address Claude review follow-ups

stats/src/components/vocabulary/VocabularyTab.tsx

@@ -47,6 +47,19 @@ export function VocabularyTab({
     if (excluded.length > 0) result = result.filter((w) => !isExcluded(w));
     return result;
   }, [words, hideNames, excluded, isExcluded]);
+  const summary = useMemo(
+    () => buildVocabularySummary(filteredWords, kanji),
+    [filteredWords, kanji],
+  );
+  const knownWordCount = useMemo(() => {
+    if (knownWords.size === 0) return 0;
+
+    let count = 0;
+    for (const w of filteredWords) {
+      if (knownWords.has(w.headword)) count += 1;
+    }
+    return count;
+  }, [filteredWords, knownWords]);
 
   if (loading) {
     return (
@@ -63,15 +76,6 @@ export function VocabularyTab({
     );
   }
 
-  const summary = buildVocabularySummary(filteredWords, kanji);
-
-  let knownWordCount = 0;
-  if (knownWords.size > 0) {
-    for (const w of filteredWords) {
-      if (knownWords.has(w.headword)) knownWordCount++;
-    }
-  }
-
   const handleSelectWord = (entry: VocabularyEntry): void => {
     onOpenWordDetail?.(entry.wordId);
   };

stats/src/hooks/useSessions.test.ts

@@ -0,0 +1,20 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import test from 'node:test';
+import { fileURLToPath } from 'node:url';
+import { toErrorMessage } from './useSessions';
+
+const USE_SESSIONS_PATH = fileURLToPath(new URL('./useSessions.ts', import.meta.url));
+
+test('toErrorMessage normalizes Error and non-Error rejections', () => {
+  assert.equal(toErrorMessage(new Error('network down')), 'network down');
+  assert.equal(toErrorMessage('bad gateway'), 'bad gateway');
+  assert.equal(toErrorMessage(503), '503');
+});
+
+test('useSessions and useSessionDetail route catch handlers through toErrorMessage', () => {
+  const source = fs.readFileSync(USE_SESSIONS_PATH, 'utf8');
+  const matches = source.match(/setError\(toErrorMessage\(err\)\)/g);
+
+  assert.equal(matches?.length, 2);
+});

stats/src/hooks/useSessions.ts

@@ -3,6 +3,10 @@ import { getStatsClient } from './useStatsApi';
 import { SESSION_CHART_EVENT_TYPES } from '../lib/session-events';
 import type { SessionSummary, SessionTimelinePoint, SessionEvent } from '../types/stats';
 
+export function toErrorMessage(err: unknown): string {
+  return err instanceof Error ? err.message : String(err);
+}
+
 export function useSessions(limit = 50) {
   const [sessions, setSessions] = useState<SessionSummary[]>([]);
   const [loading, setLoading] = useState(true);
@@ -21,7 +25,7 @@ export function useSessions(limit = 50) {
       })
       .catch((err) => {
         if (cancelled) return;
-        setError(err.message);
+        setError(toErrorMessage(err));
       })
       .finally(() => {
         if (cancelled) return;
@@ -77,7 +81,7 @@ export function useSessionDetail(sessionId: number | null) {
       })
       .catch((err) => {
         if (cancelled) return;
-        setError(err.message);
+        setError(toErrorMessage(err));
       })
       .finally(() => {
         if (cancelled) return;

stats/src/lib/vocabulary-tab.test.ts

@@ -1,11 +1,10 @@
 import assert from 'node:assert/strict';
 import fs from 'node:fs';
-import path from 'node:path';
 import test from 'node:test';
+import { fileURLToPath } from 'node:url';
 
-const VOCABULARY_TAB_PATH = path.resolve(
-  import.meta.dir,
-  '../components/vocabulary/VocabularyTab.tsx',
+const VOCABULARY_TAB_PATH = fileURLToPath(
+  new URL('../components/vocabulary/VocabularyTab.tsx', import.meta.url),
 );
 
 test('VocabularyTab declares all hooks before loading and error early returns', () => {
@@ -20,3 +19,16 @@ test('VocabularyTab declares all hooks before loading and error early returns',
 
   assert.deepEqual(hooksAfterLoadingGuard ?? [], []);
 });
+
+test('VocabularyTab memoizes summary and known-word aggregate calculations', () => {
+  const source = fs.readFileSync(VOCABULARY_TAB_PATH, 'utf8');
+
+  assert.match(
+    source,
+    /const summary = useMemo\([\s\S]*buildVocabularySummary\(filteredWords, kanji\)[\s\S]*\[filteredWords, kanji\][\s\S]*\);/,
+  );
+  assert.match(
+    source,
+    /const knownWordCount = useMemo\(\(\) => \{[\s\S]*for \(const w of filteredWords\) \{[\s\S]*knownWords\.has\(w\.headword\)[\s\S]*\}\s*return count;\s*\}, \[filteredWords, knownWords\]\);/,
+  );
+});