fix(web): handle null values in permissionMode and modelMode using nullish coalescing

The settings popup in HappyComposer did not show any option selected when
permissionMode or modelMode was null, since destructuring defaults only
handle undefined, not null. This fix uses the ?? operator to properly
handle both null and undefined cases, falling back to 'default' in both.

web/src/components/AssistantChat/HappyComposer.tsx

@@ -59,8 +59,8 @@ export function HappyComposer(props: {
 }) {
     const {
         disabled = false,
-        permissionMode = 'default',
-        modelMode = 'default',
+        permissionMode: rawPermissionMode,
+        modelMode: rawModelMode,
         active = true,
         thinking = false,
         agentState,
@@ -73,6 +73,10 @@ export function HappyComposer(props: {
         autocompleteSuggestions = defaultSuggestionHandler
     } = props
 
+    // Use ?? to handle both null and undefined (destructuring defaults only work for undefined)
+    const permissionMode = rawPermissionMode ?? 'default'
+    const modelMode = rawModelMode ?? 'default'
+
     const api = useAssistantApi()
     const composerText = useAssistantState(({ composer }) => composer.text)
     const threadIsRunning = useAssistantState(({ thread }) => thread.isRunning)