fix(storage): secrets cannot be cleared on edit, stale SSH secrets stay after SSH is turned off, and a failed save restores only part of them #1311

Problem

lib/features/connections/connection_edit_secrets.dart, lib/core/storage/connection_secrets_store.dart, lib/core/storage/local_db.dart (updateConnection):

1. No way to clear. A blank password / SSH field means keep the previous one, and the form never shows the stored value, so a saved password cannot be removed (the database moved to trust authentication, the SSH key is gone).
2. Stale SSH secrets. writeForConnection writes SSH secrets only when one of them is non-null. Turning SSH off leaves the old password, private key and passphrase in the keyring until the connection is deleted.
3. Partial rollback. When writing secrets fails, updateConnection restores the previous row and the password / connection string, but not the SSH secrets, which may already be partly overwritten (writeSshSecretsForConnection writes four keys one by one).
4. writeSshSecretsForConnection writes all four keys: a caller that passes only one nulls the other three.

Scope

• An explicit Remove saved password / Remove saved SSH secrets action in the forms (a tri-state: keep / replace / clear).
• Delete SSH secrets when the tunnel is disabled.
• Snapshot and restore all secrets (database and SSH) on a failed update.
• Tests with a fake backend: clear, disable SSH, failing write mid-way.

Acceptance

• A saved password can be removed from the edit form, and the next connect sends none.
• After disabling SSH no SSH secret is left in the store.
• A failed update leaves every secret as it was.