ux(connections): a failed save of a new connection discards everything the user typed #1758

Problem

The creation flows (`promptCreateConnection`, `showNewConnectionUrlDialog`, `showSqliteConnectionForm`) close their dialog and return a `ConnectionRow`; the caller saves it afterwards (`saveNewConnection`, #1757). When the save fails (typically the system keyring refusing the password, seen on macOS), the row is rolled back and the user gets a toast, but the form is already gone. A long form (host, SSH tunnel, certificates, password) has to be typed again to retry.

Proposal

Keep the form open until the save succeeded: either save inside the dialog flow (show the error in the form and let the user press Save again), or reopen the form prefilled with the returned row on failure.

Where

• `lib/features/connections/connection_creation_flow.dart` (`saveNewConnection`, `promptCreateConnection`)
• `lib/features/main_screen/main_screen_dialogs.dart`, `lib/features/connections/connections_panel.dart` (callers)

Found in the review of #1757.