fix(settings): add loading feedback to account create/edit dialogs

[pringelmann]

• Resolves: [Bug]: accounts -> new account UX improvable #46783

Summary

While an account is being created or edited, the dialog now shows clear progress: the submit button gets a spinner and a busy label, the form is locked, and the close button is disabled until the request settles. The submit button uses aria-disabled rather than disabled so the loading state stays announced to screen readers, with a guard preventing re-submit.

Note: the save endpoints used in the recording below are intentionally delayed to demonstrate the fix.

Before:

user-modal-save_before.mp4

After:

user-modal-save_after.mp4

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly generated using AI

Tests authored with Claude Code (Opus). All tests were manually reviewed.

[pringelmann]

/compile amend