fix(app): the short reading's nits on the device

- Deleting an identified account whose security can't be read (the
  instance out of reach) says so and deletes nothing; it went on to a
  deletion by the session alone, refused for an account with a password,
  and the person saw a spinner, then nothing. A refusal still leaves the
  session's own deletion.
- InstanceChoice's sharesStorage is required, so no app leaves it out and
  removes a folder another instance uses.
- A PasswordUnlock forgotten is spent: its secrets read again fail
  (StateError) rather than make a proof or a wrapping of zeros.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/test/password/bloc/password_sign_in_cubit_test.dart

@@ -53,9 +53,8 @@ void main() {
     finished: finished,
   );
 
-  bool wiped(PasswordUnlock unlock) =>
-      unlock.srpSecret.every((byte) => byte == 0) &&
-      unlock.keyEncryptionKey.every((byte) => byte == 0);
+  /// Whether [unlock] was wiped, and so spent (the short reading).
+  bool wiped(PasswordUnlock unlock) => unlock.isForgotten;
 
   test(
     'wipes what the password unlocked once signed in, once another '

packages/halkyon_app/test/password/data/password_unlock_test.dart

@@ -16,9 +16,34 @@ void main() {
       ),
       srpSecret: Uint8List.fromList(List.filled(32, 7)),
       keyEncryptionKey: Uint8List.fromList(List.filled(32, 9)),
-    )..forget();
+    );
+    final x = unlock.srpSecret;
+    final key = unlock.keyEncryptionKey;
 
-    expect(unlock.srpSecret, everyElement(0));
-    expect(unlock.keyEncryptionKey, everyElement(0));
+    unlock.forget();
+
+    expect(x, everyElement(0));
+    expect(key, everyElement(0));
+  });
+
+  test('a forgotten unlock is spent: its secrets are read no more, so no '
+      'proof is made of zeros (the short reading)', () {
+    final unlock = PasswordUnlock(
+      salt: Uint8List(16),
+      costs: protocol.PasswordCosts(
+        algorithm: 'argon2id13',
+        opsLimit: 3,
+        memLimit: 64 * 1024 * 1024,
+      ),
+      srpSecret: Uint8List.fromList(List.filled(32, 7)),
+      keyEncryptionKey: Uint8List.fromList(List.filled(32, 9)),
+    );
+    expect(unlock.isForgotten, isFalse);
+
+    unlock.forget();
+
+    expect(unlock.isForgotten, isTrue);
+    expect(() => unlock.srpSecret, throwsStateError);
+    expect(() => unlock.keyEncryptionKey, throwsStateError);
   });
 }