feat: secure account sharing via public-key encryption (#107)

Replace the insecure plaintext QR export/import flow with an
end-to-end-encrypted account-transfer mechanism:

- Receiver generates an ephemeral X25519 key pair (20-minute lifetime,
  stored in the new share_keys DB table at schema v31) and displays it
  as a QR code (sharedinbox.de:pubkey:v1:…).
- Sender scans the public-key QR, selects accounts (or auto-selects
  when only one exists), encrypts them with ECIES (X25519-ECDH +
  HKDF-SHA256 + AES-256-GCM) and displays an encrypted QR
  (sharedinbox.de:encrypted-accounts:v1:…).
- Receiver scans the encrypted QR, decrypts, verifies the 20-minute
  expiry and MAC authentication tag, then imports the accounts.

New screens: AccountReceiveScreen (/accounts/receive) and
AccountSendScreen (/accounts/send), accessible from the account-list
drawer and per-account popup menu respectively.

Remove the old insecure AccountExportScreen and AccountImportScreen.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/check_coverage.dart

@@ -15,6 +15,7 @@ const _noCode = {
   'lib/core/repositories/draft_repository.dart',
   'lib/core/repositories/email_repository.dart',
   'lib/core/repositories/mailbox_repository.dart',
+  'lib/core/repositories/share_key_repository.dart',
   'lib/core/repositories/sync_log_repository.dart',
   'lib/core/repositories/undo_repository.dart',
   'lib/core/repositories/search_history_repository.dart',
@@ -32,9 +33,9 @@ const _excluded = {
   'lib/di.dart',
   'lib/main.dart',
   'lib/ui/router.dart',
-  'lib/ui/screens/account_export_screen.dart',
-  'lib/ui/screens/account_import_screen.dart',
   'lib/ui/screens/account_list_screen.dart',
+  'lib/ui/screens/account_receive_screen.dart',
+  'lib/ui/screens/account_send_screen.dart',
   'lib/ui/screens/add_account_screen.dart',
   'lib/ui/screens/address_emails_screen.dart',
   'lib/ui/screens/changelog_screen.dart',
@@ -63,6 +64,7 @@ const _excluded = {
   'lib/data/repositories/account_repository_impl.dart',
   'lib/data/repositories/email_repository_impl.dart',
   'lib/data/repositories/mailbox_repository_impl.dart',
+  'lib/data/repositories/share_key_repository_impl.dart',
   'lib/data/repositories/sync_log_repository_impl.dart',
   'lib/data/repositories/undo_repository_impl.dart',
   'lib/data/repositories/search_history_repository_impl.dart',