fix: add missing Link import in SettingsPage

Critical bug fix:
- Added missing "import { Link } from 'react-router-dom'"
- Caused Settings page to crash on render
- Route /settings now works again

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

frontend/src/pages/SettingsPage.jsx

@@ -1,4 +1,5 @@
 import { useState } from 'react'
+import { Link } from 'react-router-dom'
 import { Save, Download, Upload, Trash2, Plus, Check, Pencil, X, LogOut, Shield, Key } from 'lucide-react'
 import { useProfile } from '../context/ProfileContext'
 import { useAuth } from '../context/AuthContext'