Portfolio: editor form with live preview #45

Part of #40. Depends on #43 and #44.

Scope

• A signed-in page, for example /learn/portfolio. Guests see a prompt to sign in.
• Sections matching the shared fields. Projects and other entries can be added, removed and reordered.
• Inline validation messages from the shared rules.
• A live preview that uses the same theme components as the public page.
• Save draft, publish and unpublish. Claim a username with an availability check.
• Phone-first layout, labelled fields, visible focus, and errors that do not rely on colour alone (see CONTRIBUTING.md).

Done when

A browser test covers: fill in the form, save, publish, and view the page at /u/<username> on desktop and mobile.