feat(web): retain author-selected link card presentation #197

Link snapshots currently have one published presentation. Authors need a persisted presentation preference that can change independently of captured page metadata.

Expected behavior:

• Default older content and omitted input to the existing cover presentation, while supporting a compact alternative.
• Preserve the choice through browser drafts, publication, subsequent edits, reading surfaces, generated share images, and portable exports/imports.
• Update presentation without re-fetching the page or replacing its captured metadata and managed media references; expose equivalent HTTP and CLI operations.
• Use the existing accessible menu system, with stable selection alignment and reliable keyboard, pointer, touch, and dismissal behavior across responsive layouts and themes.

Implementation

#198