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/components/GuideScreenshot.tsx

@@ -0,0 +1,26 @@
+interface GuideScreenshotProps {
+  src: string;
+  alt: string;
+  caption?: string;
+  mobile?: boolean;
+}
+
+export const GuideScreenshot = ({ src, alt, caption, mobile }: GuideScreenshotProps) => {
+  return (
+    <figure className={`my-6 ${mobile ? 'max-w-[375px]' : 'max-w-4xl'} mx-auto`}>
+      <div className="rounded-lg overflow-hidden border border-white/10 shadow-lg shadow-black/20">
+        <img
+          src={src}
+          alt={alt}
+          loading="lazy"
+          className="w-full h-auto"
+        />
+      </div>
+      {caption && (
+        <figcaption className="mt-2 text-center text-sm text-titanio/70 italic">
+          {caption}
+        </figcaption>
+      )}
+    </figure>
+  );
+};