fix(editor): default sidebar to build tab, drop to select on panel close (#495)

The AI/chat tab was the default panel when opening a project; build is
the expected starting point. Also, collapsing the sidebar (rail click or
drag-collapse on desktop, sheet close on mobile) left the last build
tool armed — it now resets to select mode.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/editor/src/components/editor/editor-layout-mobile.tsx

@@ -159,6 +159,11 @@ export function EditorLayoutMobile({
       const expandedThreshold = Math.max(SHEET_HANDLE_PX, defaultPx * 0.5)
       if (current > expandedThreshold) {
         sheetRef.current?.snapTo(SHEET_HANDLE_PX)
+        // Closing the sheet disarms any build tool back to select
+        const { mode, setMode } = useEditor.getState()
+        if (mode === 'build') {
+          setMode('select')
+        }
       } else {
         sheetRef.current?.snapTo(defaultPx)
       }

packages/editor/src/components/editor/editor-layout-v2.tsx

@@ -52,6 +52,15 @@ function LeftColumn({
     }
   }, [activePanel])
 
+  // Closing (collapsing) the sidebar disarms any build tool back to select
+  useEffect(() => {
+    if (!isCollapsed) return
+    const { mode, setMode } = useEditor.getState()
+    if (mode === 'build') {
+      setMode('select')
+    }
+  }, [isCollapsed])
+
   const handleResizerDown = useCallback(
     (e: React.PointerEvent) => {
       e.preventDefault()

packages/editor/src/store/use-editor.tsx

@@ -66,7 +66,7 @@ import {
 } from '../lib/snapping-mode'
 import useInteractionScope from './use-interaction-scope'
 
-const DEFAULT_ACTIVE_SIDEBAR_PANEL = 'ai'
+const DEFAULT_ACTIVE_SIDEBAR_PANEL = 'build'
 const DEFAULT_FLOORPLAN_PANE_RATIO = 0.5
 const MIN_FLOORPLAN_PANE_RATIO = 0.15
 const MAX_FLOORPLAN_PANE_RATIO = 0.85