style: Restore password entry width
Commit removed the fixed width that is used by polkit/keyring prompts, with the result that the entries now start at their natural size, and grow as text is entered.
Restore the original behavior, but nest the style under .prompt-dialog-password-layout (polkit/keyring) to better differentiate it from entries under ..prompt-dialog-password-grid (mount operations).
Fixes: 8d065b55 ("style: Clean up dialogs stylesheet to fix layout issues")