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.

android/app/src/main/java/io/narl/protonstream/ui/AccountUi.kt

@@ -332,10 +332,12 @@ private fun VerificationDialog(url: String, onToken: (String) -> Unit, onDismiss
     }
 }
 
-/** 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::sign_in`).
+ */
 private const val SECURITY_KEY_NOTE =
-    "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."
 
 /** Installed once per page, at start and again at finish in case the first ran too early. */
 private const val VERIFICATION_BRIDGE = """