- Add error display in renderChannelConfigSection
- Show props.lastError in danger callout when save fails
- Fixes#48923 where users couldn't see why config save failed
Root cause:
The saveConfig() function sets state.lastError on failure, but the
Channels UI never displayed it. Users saw unresponsive page with no
feedback, making it impossible to diagnose config save failures.
Fix:
1. Check props.lastError in renderChannelConfigSection
2. Display error in danger callout when present
3. Error clears on next successful save