Keep account form values when adding an account fails

[tsouth89]

Problem

The directory-account form is reset even when addDirectoryAccount rejects. A user who enters a label and directory, hits an error, and wants to retry must re-enter both values.

Where

apps/desktop-tauri/src/surfaces/settings/providers/AccountsPanel.tsx

The helper catches the rejection, but the submit path calls resetForm() unconditionally afterward.

Expected behavior

• Clear the form only after a successful account creation.
• Preserve the entered label and directory on failure.
• Keep the existing error message visible so the user can correct and retry.

Tests

Add a rejected-promise regression test that verifies both fields are preserved, plus a success case that verifies they are cleared.