fix: settings page defaults to Profilo tab, banner links to Provider AI

- Change default activeSection from 'piano' to 'profilo'
- Support ?tab= query param to deep-link into settings sections
- Update missing API key banner to link to /settings?tab=ai

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/SettingsPage.jsx

@@ -1,5 +1,5 @@
 import React, { useState, useEffect, useRef } from 'react'
-import { useNavigate } from 'react-router-dom'
+import { useNavigate, useSearchParams } from 'react-router-dom'
 import { api } from '../api'
 import { useAuth } from '../AuthContext'
 
@@ -126,7 +126,8 @@ const SECTIONS = [
 export default function SettingsPage() {
   const { user, isPro } = useAuth()
   const navigate = useNavigate()
-  const [activeSection, setActiveSection] = useState('piano')
+  const [searchParams] = useSearchParams()
+  const [activeSection, setActiveSection] = useState(searchParams.get('tab') || 'profilo')
   const [aiValues, setAiValues] = useState({
     llm_provider: 'claude', llm_api_key: '', llm_model: '', llm_base_url: '',
     image_provider: 'dalle', image_api_key: '', image_base_url: '',