fix: align payloadCaptureMode default with agent (BOTH, not NONE)

Server defaultConfig() and UI fallbacks returned "NONE" for payload
capture, but the agent defaults to "BOTH". This caused unwanted
reconfiguration when users saved other settings — payload capture
would silently change from the agent's default BOTH to NONE.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Admin/AppConfigDetailPage.tsx

@@ -99,7 +99,7 @@ export default function AppConfigDetailPage() {
         applicationLogLevel: config.applicationLogLevel ?? 'INFO',
         agentLogLevel: config.agentLogLevel ?? 'INFO',
         engineLevel: config.engineLevel ?? 'REGULAR',
-        payloadCaptureMode: config.payloadCaptureMode ?? 'NONE',
+        payloadCaptureMode: config.payloadCaptureMode ?? 'BOTH',
         metricsEnabled: config.metricsEnabled,
         samplingRate: config.samplingRate,
         compressSuccess: config.compressSuccess,
@@ -115,7 +115,7 @@ export default function AppConfigDetailPage() {
       applicationLogLevel: config.applicationLogLevel ?? 'INFO',
       agentLogLevel: config.agentLogLevel ?? 'INFO',
       engineLevel: config.engineLevel ?? 'REGULAR',
-      payloadCaptureMode: config.payloadCaptureMode ?? 'NONE',
+      payloadCaptureMode: config.payloadCaptureMode ?? 'BOTH',
       metricsEnabled: config.metricsEnabled,
       samplingRate: config.samplingRate,
       compressSuccess: config.compressSuccess,