fix(settings): stop create-admin sheet crash after save

[JohnChantz]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx, where "xxx" is the issue number)

What was broken

Clicking Create on Settings → Admin Users → "Create new admin user" saved the admin, then crashed the page with a Next.js overlay (Maximum update depth exceeded in AlertProvider).

Cause

postNewAdminUser succeeded. Closing the sheet ran a useEffect that treated the dirty form as an unsaved discard, called addAlert on every render (unstable setOpen in the effect deps), and looped. Inputs also had no defaultValues, so they flipped between uncontrolled and controlled.

Fix

• Keep the create form controlled with empty defaults
• Confirm discard from onOpenChange instead of an effect
• Reset, then close, after a successful create
• Wrap admin API failures as plain Errors so Server Actions can serialize them
• Subscribe AlertProvider to isOpen instead of the undefined open identifier

Test plan

• Vitest: AddAdminSheet controlled defaults and successful create without the unsaved-changes dialog
• Vitest: withAdminApiError serializes axios-like failures and rethrows Next redirects
• Manual: log in, create uiadmin_fixed, confirm success toast, table row, and no overlay

create_admin_user_success.mp4

Other information:

Targets ui-rewrite.