Fix suggestions fetch using wrong API base URL

Fallback was http://localhost:8000/api instead of /api, causing silent
failure in containerized deployments.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/hooks/useSuggestions.js

@@ -1,6 +1,6 @@
 import { useState, useEffect } from 'react'
 
-const API_BASE = import.meta.env.VITE_API_URL || 'http://localhost:8000/api'
+const API_BASE = import.meta.env.VITE_API_URL || '/api'
 
 export function useSuggestions() {
   const [suggestions, setSuggestions] = useState({ authors: [], books: [], chapters: [], reviewers: [] })