Always show a panel (#1988)

## Summary

**Fixed:** Invalid layout state where only left and right sidebars are visible (no main content area).

### Changes Made:

**1. `frontend/src/stores/useLayoutStore.ts`**
- Added `setMainPanelVisible` setter function to the type and implementation
- Added `useIsRightMainPanelVisible` derived selector hook that checks if Changes/Logs/Preview panel is visible

**2. `frontend/src/components/ui-new/containers/WorkspacesLayout.tsx`**
- Imported `useIsRightMainPanelVisible` from the layout store
- Added `setMainPanelVisible` to the destructured store values
- Replaced verbose `isChangesMode || isLogsMode || isPreviewMode` checks with the helper
- Added guard effect that ensures the left main panel (chat) is visible when the right main panel is hidden - this prevents the invalid state after page reload

The guard effect automatically opens the main panel whenever the layout would otherwise show only sidebars:

```typescript
// Ensure left main panel (chat) is visible when right main panel is hidden
// This prevents invalid state where only sidebars are visible after page reload
useEffect(() => {
  if (!isMainPanelVisible && !isRightMainPanelVisible) {
    setMainPanelVisible(true);
  }
}, [isMainPanelVisible, isRightMainPanelVisible, setMainPanelVisible]);
```

frontend/src/components/ui-new/containers/WorkspacesLayout.tsx

@@ -40,7 +40,10 @@ import {
   useExpandedAll,
   PERSIST_KEYS,
 } from '@/stores/useUiPreferencesStore';
-import { useLayoutStore } from '@/stores/useLayoutStore';
+import {
+  useLayoutStore,
+  useIsRightMainPanelVisible,
+} from '@/stores/useLayoutStore';
 import { useDiffViewStore } from '@/stores/useDiffViewStore';
 import { CommandBarDialog } from '@/components/ui-new/dialogs/CommandBarDialog';
 import { useCommandBarShortcut } from '@/hooks/useCommandBarShortcut';
@@ -281,8 +284,12 @@ export function WorkspacesLayout() {
     setLogsMode,
     resetForCreateMode,
     setSidebarVisible,
+    setMainPanelVisible,
   } = useLayoutStore();
 
+  // Derived state: right main panel (Changes/Logs/Preview) is visible
+  const isRightMainPanelVisible = useIsRightMainPanelVisible();
+
   // Read persisted draft for sidebar placeholder (works outside of CreateModeProvider)
   const { scratch: draftScratch } = useScratch(
     ScratchType.DRAFT_WORKSPACE,
@@ -435,16 +442,13 @@ export function WorkspacesLayout() {
   // Ref to Allotment for programmatic control
   const allotmentRef = useRef<AllotmentHandle>(null);
 
-  // Reset Allotment sizes when changes, logs, or preview panel becomes visible
+  // Reset Allotment sizes when right main panel becomes visible
   // This re-applies preferredSize percentages based on current window size
   useEffect(() => {
-    if (
-      (isChangesMode || isLogsMode || isPreviewMode) &&
-      allotmentRef.current
-    ) {
+    if (isRightMainPanelVisible && allotmentRef.current) {
       allotmentRef.current.reset();
     }
-  }, [isChangesMode, isLogsMode, isPreviewMode]);
+  }, [isRightMainPanelVisible]);
 
   // Reset changes and logs mode when entering create mode
   useEffect(() => {
@@ -453,12 +457,20 @@ export function WorkspacesLayout() {
     }
   }, [isCreateMode, resetForCreateMode]);
 
-  // Show sidebar when no panel is open
+  // Show sidebar when right main panel is hidden
   useEffect(() => {
-    if (!isChangesMode && !isLogsMode && !isPreviewMode) {
+    if (!isRightMainPanelVisible) {
       setSidebarVisible(true);
     }
-  }, [isChangesMode, isLogsMode, isPreviewMode, setSidebarVisible]);
+  }, [isRightMainPanelVisible, setSidebarVisible]);
+
+  // Ensure left main panel (chat) is visible when right main panel is hidden
+  // This prevents invalid state where only sidebars are visible after page reload
+  useEffect(() => {
+    if (!isMainPanelVisible && !isRightMainPanelVisible) {
+      setMainPanelVisible(true);
+    }
+  }, [isMainPanelVisible, isRightMainPanelVisible, setMainPanelVisible]);
 
   // Command bar keyboard shortcut (CMD+K)
   const handleOpenCommandBar = useCallback(() => {
@@ -748,7 +760,7 @@ export function WorkspacesLayout() {
         <Allotment.Pane
           minSize={300}
           preferredSize={changesPanelWidth}
-          visible={isChangesMode || isLogsMode || isPreviewMode}
+          visible={isRightMainPanelVisible}
         >
           <div className="h-full overflow-hidden">
             {isChangesMode && (

frontend/src/stores/useLayoutStore.ts

@@ -26,6 +26,7 @@ type LayoutState = {
   setLogsMode: (value: boolean) => void;
   setPreviewMode: (value: boolean) => void;
   setSidebarVisible: (value: boolean) => void;
+  setMainPanelVisible: (value: boolean) => void;
 
   // Preview actions
   triggerPreviewRefresh: () => void;
@@ -168,6 +169,8 @@ export const useLayoutStore = create<LayoutState>()(
 
       setSidebarVisible: (value) => set({ isSidebarVisible: value }),
 
+      setMainPanelVisible: (value) => set({ isMainPanelVisible: value }),
+
       triggerPreviewRefresh: () =>
         set((s) => ({ previewRefreshKey: s.previewRefreshKey + 1 })),
 
@@ -200,3 +203,7 @@ export const useIsGitPanelVisible = () =>
 export const useIsChangesMode = () => useLayoutStore((s) => s.isChangesMode);
 export const useIsLogsMode = () => useLayoutStore((s) => s.isLogsMode);
 export const useIsPreviewMode = () => useLayoutStore((s) => s.isPreviewMode);
+
+// Derived selector: true when right main panel content is visible (Changes/Logs/Preview)
+export const useIsRightMainPanelVisible = () =>
+  useLayoutStore((s) => s.isChangesMode || s.isLogsMode || s.isPreviewMode);