feat: implement 33 nice-to-have features + fix 37 code review bugs

5 SDD batches archived:
- Batch 1: UI Polish (10 features, 14 tasks)
- Batch 2: Study System (8 features, 23 tasks)
- Batch 3: Infrastructure (5 features, 22 tasks)
- Batch 4: AI Advanced (5 features, 30 tasks) — RAG with @xenova/transformers
- Batch 5: Core Features (5 features, 19 tasks)

37 bugs fixed from comprehensive code review (11 CRITICAL, 12 HIGH, 14 MEDIUM/LOW):
- SSE streaming now works (event.token check)
- API keys no longer exposed via GET /api/models
- FTS5 injection sanitized
- DB backup/restore with admin auth
- Buddy mode wired (buddy_meta column)
- Exam auto-submit stale closure fixed
- CSS variables aligned with design tokens
- Progress data corruption fixed
- WebSocket protocol auto-detection
- Tests infrastructure completed (vitest + node:test)

client/src/hooks/useSearch.js

@@ -0,0 +1,44 @@
+import { useState, useEffect, useCallback, useRef } from 'react';
+import { searchApi } from '../lib/api';
+
+export default function useSearch() {
+  const [query, setQuery] = useState('');
+  const [results, setResults] = useState([]);
+  const [loading, setLoading] = useState(false);
+  const debounceRef = useRef(null);
+
+  const clear = useCallback(() => {
+    setQuery('');
+    setResults([]);
+    setLoading(false);
+  }, []);
+
+  useEffect(() => {
+    if (debounceRef.current) {
+      clearTimeout(debounceRef.current);
+    }
+    const trimmed = query.trim();
+    if (!trimmed) {
+      setResults([]);
+      setLoading(false);
+      return;
+    }
+    setLoading(true);
+    debounceRef.current = setTimeout(async () => {
+      try {
+        const data = await searchApi(trimmed);
+        setResults(data || []);
+      } catch (err) {
+        console.error('[useSearch] error:', err.message);
+        setResults([]);
+      } finally {
+        setLoading(false);
+      }
+    }, 300);
+    return () => {
+      if (debounceRef.current) clearTimeout(debounceRef.current);
+    };
+  }, [query]);
+
+  return { query, setQuery, results, loading, clear };
+}