Fix: Awaiting Response Category cannot be unset

[biast12]

Description

Once a panel's Awaiting Response Category was set, there was no way to unset it — the dropdown had no "None" entry, so the only escape was deleting the Discord category.

The backend already supported unset (pending_category is nullable, and validatePendingCategory returns nil early when the value is nil), so this was a UI gap.

It also caused a save lock-out: validation.go:277 rejects a non-nil value for non-premium guilds, but the dropdown was disabled for those guilds. A server whose premium lapsed with a category set could not save any edit to that panel, with no way to clear the offending field.

Type of Change

• Bug fix
• New feature
• Breaking change

Testing

Set Awaiting Response Category to the none value

Checklist

• My code follows the style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works