Add UI to edit a dynamic QR code's destination #3

Dynamic QR codes keep the same short URL so their destination can change after printing. Today the only way to change it is the API:

PATCH /api/qr-codes/:id   { "contentPayload": { "url": "https://new.example" }, "name": "...", "designConfig": {...} }

The React UI (SelectedRecord in src/client/App.tsx) shows the record's short URL, encoded payload and stats, but it has no edit form. The main benefit of a dynamic code isn't available from the UI.

Proposed:

• An "Edit" action on the selected record that loads the current contentPayload (and optionally name/design) into the existing payload fields.
• Submit via PATCH /api/qr-codes/:id (send x-admin-token when configured), then refresh the record and the preview.
• Show the audit history from GET /api/qr-codes/:id (auditEvents) so users can see earlier destinations. As of PR Fix audit snapshots, landing feedback forms, PDF/EPS errors, gradient on eyes #1, audit entries are immutable snapshots.
• Make it clear that for dynamic codes the printed QR does not change; for static codes, editing changes the encoded payload, so the code has to be reprinted.

Found during an article review.