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>

pubspec.yaml

@@ -43,6 +43,12 @@ dependencies:
   # QR code generation for account sharing
   qr_flutter: ^4.1.0
 
+  # Public-key encryption for secure account sharing (ECIES: X25519 + AES-256-GCM)
+  cryptography: ^2.7.0
+
+  # QR code scanning (camera) for secure account import
+  mobile_scanner: ^5.0.0
+
   # HTML rendering for email bodies
   webview_flutter: ^4.0.0
   url_launcher: ^6.3.2