fix(core,app,android): keep the password out of storage and refuse key-only 2FA

Moves to proton-sdk / proton-drive-rs 0.7.3.

The stored Proton session now holds, per account key, the passphrase derived
from the mailbox password at sign-in, and resumes through
`ProtonDriveClient::with_key_passphrases`. The mailbox password — on a
single-password account, the login password — and the key salts are no longer
stored. A session in the earlier shape, which only development builds of
1.2.0 wrote, reads as signed out: signing in again replaces it.

An account whose only second factor is a security key is refused at sign-in
with a reason, rather than asked for a code that can only fail as a wrong one.
The code prompts now say plainly that security keys are not supported.

crates/pstr-app/src/ui/account.rs

@@ -208,10 +208,10 @@ fn code_form(ui: &mut egui::Ui, panel: &mut AccountPanel, actions: &mut Vec<Acti
     );
     field.request_focus();
     ui.add_space(theme::space::S);
-    // Security keys need WebAuthn, which the sign-in here does not speak.
+    // Security keys need WebAuthn, which the sign-in here does not speak. An
+    // account with nothing else is refused before this form (`AccountStore`).
     ui.label(ui::muted(
-        "Security keys are not supported. If your account has only a security key, add an \
-         authenticator app in your Proton account settings to sign in here.",
+        "Security keys are not supported here — use the code from your authenticator app.",
     ));
     ui.add_space(theme::space::L);
     error_line(ui, panel);