Add Export account screen (QR code + copy-to-clipboard) and Import
account screen (paste JSON code) so users can transfer IMAP/JMAP
account configuration to another device without re-entering every field.
- Account list popup: "Export account" opens a QR code with a password
warning and a copy-code button.
- Add Account screen: "Import account" button opens the import flow
where pasting the exported JSON pre-fills the account and one tap
saves it with a fresh generated ID.
- New routes: /accounts/:id/export and /accounts/import.
- Widget tests cover export display, import parsing, validation,
and the happy-path save-and-navigate flow.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
