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>

apps/todo/halkyon_todo_flutter/test/app/todo_app_test.dart

@@ -36,6 +36,7 @@ void main() {
       store,
       builtIn: builtIn,
       forget: (instance) async {},
+      sharesStorage: (a, b) => false,
     );
     await choice.load(usedBefore: usedBefore);
     await signedOut.write(signedOut: wasSignedOut);