frontend: API error messages are silently swallowed in channel creation modal

[Fullstop000]

Bug description

When the server rejects a channel creation request, the frontend shows no error message to the user. The modal stays open and the user has no idea what went wrong.

Steps to reproduce

1. Click "Add channel" in the sidebar
2. Enter an invalid channel name (e.g. "space channel")
3. Click "Create Channel"
4. Observe: the modal stays open, no error is shown
5. Check browser console: Failed to load resource: the server responded with a status of 400

Expected behavior

The modal should display the server error message (e.g. "channel name can only contain lowercase letters, numbers, hyphens, and underscores") so the user knows what to fix.

Also affects

• Duplicate channel names (409 Conflict → no error shown)
• Likely affects other forms too (team creation, settings, etc.)

Evidence

Browser network log:

POST /api/channels → 400 (2ms, 94B)

Console log shows the error but no UI feedback is rendered.

[Fullstop000]

Verified: frontend DOES show API errors via FormError component. Initial test missed it due to accessibility snapshot limitations. Live test confirms error text renders correctly.