feat(accounts): share account settings via QR code / JSON export (#66)

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>

pubspec.yaml

@@ -40,6 +40,9 @@ dependencies:
   open_filex: ^4.6.0
   mime: ^2.0.0
 
+  # QR code generation for account sharing
+  qr_flutter: ^4.1.0
+
   # HTML rendering for email bodies
   webview_flutter: ^4.0.0
   url_launcher: ^6.3.2