[Studio][Bug] Cancelled create-user dialog keeps the password and admin switch for the next attempt

## Problem

The create-user dialog on the Studio user management page (`web/src/pages/studio/UserManagement.tsx`) keeps its form mounted across open/close cycles. The cancel path (`onCancel`, line ~513) only closes the dialog: there is no `resetFields()` and no `destroyOnHidden`. The form is only reset on the create-success path (`createUser`, lines ~179-180) and in the reset-password dialog of the same file.

## Evidence

1. Open the create dialog, type a username, an initial password and enable the admin switch.
2. Cancel (or press Esc / click the mask).
3. Reopen the dialog: the username, the initial password and the admin switch are still filled.
4. Change only the username and submit: `createStudioUser` is called with the previous attempt's initial password and `admin: true` from the cancelled attempt.

A regression test demonstrating the residue on the unfixed code lives in the linked PR.

## Impact

An operator who cancels an attempt and later creates a different user can unknowingly create an admin account and reuse a password they believed was discarded. The created account is elevated and its credential is not the one the operator chose for it.

## Expected behavior

Dismissing the dialog without creating resets the form to its initial values, so the next attempt starts from a clean username, password and admin switch.

## Related work

The same page's reset-password dialog already resets on cancel, and the create-success path resets the form; sibling create dialogs reset on cancel (`cluster/index.tsx` NameServer create, `instance/index.tsx` create/edit). Issue #4251 records the same residue class for the maintenance-window dialog on the system alerts page.

## PR

Fix: #4253.

Start in web/src/pages/studio/UserManagement.tsx, reading the create-user dialog's onCancel path around line 513 and the createUser reset around lines 179-180. The linked regression test demonstrates the residue; done means dismissing the dialog leaves the next attempt with clean username, password, and admin values.

Written by the indexing model from the issue text.