feat: add guide page foundation and routing (refs #203)

- Create GuidePage with responsive layout (sticky TOC sidebar desktop, collapsible accordion mobile)
- Add GuideTableOfContents with scroll-based active section tracking
- Create GuideScreenshot and GuideTable shared components
- Add guideTypes.ts with section metadata for all 10 sections
- Add lazy-loaded /guide route in App.tsx with public access
- Placeholder section components for all 10 guide sections

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

frontend/src/pages/GuidePage/sections/SettingsSection.tsx

@@ -0,0 +1,8 @@
+export const SettingsSection = () => {
+  return (
+    <section id="settings" className="py-8 border-b border-white/5 scroll-mt-24">
+      <h2 className="text-2xl font-bold text-avus mb-4">8. Settings</h2>
+      <p className="text-titanio/70">Content loading in Milestone 3...</p>
+    </section>
+  );
+};