fix(ui): display error message when channel config save fails

- 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

ui/src/ui/views/channels.config.ts

@@ -118,6 +118,7 @@ export function renderChannelConfigForm(props: ChannelConfigFormProps) {
 export function renderChannelConfigSection(params: { channelId: string; props: ChannelsProps }) {
   const { channelId, props } = params;
   const disabled = props.configSaving || props.configSchemaLoading;
+  const hasError = Boolean(props.lastError);
   return html`
     <div style="margin-top: 16px;">
       ${
@@ -134,6 +135,15 @@ export function renderChannelConfigSection(params: { channelId: string; props: C
               onPatch: props.onConfigPatch,
             })
       }
+      ${
+        hasError
+          ? html`
+              <div class="callout danger" style="margin-top: 12px;">
+                ${props.lastError}
+              </div>
+            `
+          : nothing
+      }
       <div class="row" style="margin-top: 12px;">
         <button
           class="btn primary"