Portfolio: public page at /u/[username] with two themes #44

Part of #40. Depends on #43.

Scope

• A server-rendered page built from published data. Drafts, suspended pages and unknown usernames show the 404 page.
• Two themes, for example 'Simple' and 'Developer', chosen by the student. Build them as components so new themes can be added later.
• Metadata: title, description and Open Graph. Add noindex unless the student turned on search visibility; list only opted-in portfolios in the sitemap.
• No third-party requests: no remote images or fonts. Show an initials avatar instead of a photo in Phase 1.
• External links use rel="noopener noreferrer".
• Cache published pages and refresh them after publish, unpublish or suspension.
• Works on a phone and with a keyboard.

Done when

npm run e2e and npm run launch-check cover a published page, a draft (404), and the noindex default.