feat(editor): build sidebar + slimmed items panel for standalone editor (#361)

Bring the open-source standalone editor closer to the community editor's
v2 sidebar without pulling in any preset/catalog infrastructure.

- Add a preset-less Build tab (apps/editor) that mirrors the community
  Build sidebar: wall, fence, slab, ceiling, roof, stair, elevator,
  door, window, column, spawn, plus the material-paint panel. Clicking a
  type activates the raw structure tool drawn with the kind's defaults.
- Wire the Build tab into both editor mount points (local + saved scene)
  and give the left rail proper image icons (Scene/Build/Items/Settings)
  instead of letter fallbacks.
- Gate the ItemsPanel Library/Community/Mine source chips and tag filter
  rows behind `showSourceFilter` / `showTagFilters` props (default true,
  so community and external consumers are unchanged). The standalone
  editor passes both off, leaving plain category tabs + full-width search.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

packages/editor/src/components/ui/sidebar/panels/items-panel/index.tsx

@@ -20,6 +20,8 @@ export function ItemsPanel({
   leadingTile,
   emptyState,
   functionTree,
+  showSourceFilter = true,
+  showTagFilters = true,
 }: {
   items?: AssetInput[]
   /** Called when the search query changes (community edition uses this for server-side search) */
@@ -41,6 +43,16 @@ export function ItemsPanel({
    * hierarchical tree browse instead of the legacy hardcoded category tabs.
    */
   functionTree?: FunctionTreeNode[]
+  /**
+   * Library/Community/Mine source chips. The open-source editor has no
+   * uploaded items (only the built-in catalog), so it hides these.
+   */
+  showSourceFilter?: boolean
+  /**
+   * Placement/functional tag filter chips under the search row. The
+   * open-source editor hides these to keep the panel to plain categories.
+   */
+  showTagFilters?: boolean
 }) {
   // When the embedder supplies a function taxonomy, the hierarchical browse
   // replaces the legacy `furnishTools` category tabs entirely.
@@ -63,6 +75,8 @@ export function ItemsPanel({
     leadingTile={leadingTile}
     onSearchChange={onSearchChange}
     searchResults={searchResults}
+    showSourceFilter={showSourceFilter}
+    showTagFilters={showTagFilters}
   />
 }
 
@@ -72,12 +86,16 @@ function LegacyItemsPanel({
   searchResults,
   leadingTile,
   emptyState,
+  showSourceFilter = true,
+  showTagFilters = true,
 }: {
   items?: AssetInput[]
   onSearchChange?: (query: string) => void
   searchResults?: AssetInput[] | null
   leadingTile?: React.ReactNode
   emptyState?: React.ReactNode
+  showSourceFilter?: boolean
+  showTagFilters?: boolean
 }) {
   const mode = useEditor((s) => s.mode)
   const catalogCategory = useEditor((s) => s.catalogCategory)
@@ -89,8 +107,11 @@ function LegacyItemsPanel({
   const [activeFunctionalTag, setActiveFunctionalTag] = useState<string | null>(null)
   // Library / Community / Mine. Default to Library so first-time users see
   // the curated catalog rather than every uploaded item; clicking the chip
-  // again clears the filter (`null` = show everything).
-  const [activeSource, setActiveSource] = useState<AssetInput['source'] | null>('library')
+  // again clears the filter (`null` = show everything). With the chips hidden
+  // there is nothing to filter by, so start unfiltered.
+  const [activeSource, setActiveSource] = useState<AssetInput['source'] | null>(
+    showSourceFilter ? 'library' : null,
+  )
   const [search, setSearch] = useState('')
   const isServerSearch = onSearchChange !== undefined
   // True when server search is active but results haven't come back yet
@@ -152,7 +173,7 @@ function LegacyItemsPanel({
   const allTags = Array.from(new Set(categoryItems.flatMap((item) => item.tags ?? [])))
   const placementTags = allTags.filter((t) => PLACEMENT_TAGS.has(t))
   const functionalTags = allTags.filter((t) => !PLACEMENT_TAGS.has(t))
-  const hasFilters = allTags.length > 1
+  const hasFilters = showTagFilters && allTags.length > 1
 
   const placementCount = (tag: string | null) =>
     categoryItems.filter((item) => {
@@ -205,9 +226,13 @@ function LegacyItemsPanel({
       <div className="flex shrink-0 flex-col gap-2 border-border/70 border-b p-2">
         <div className="flex items-center gap-1.5">
           {/* Search and source filter take 50/50 of the row. `min-w-0` on
-              both sides lets each half shrink to fit when the panel narrows. */}
+              both sides lets each half shrink to fit when the panel narrows.
+              With the source chips hidden, search spans the full row. */}
           <input
-            className="w-1/2 min-w-0 shrink-0 rounded-lg bg-muted px-2.5 py-1.5 text-xs placeholder:text-muted-foreground focus:outline-none"
+            className={cn(
+              'min-w-0 shrink-0 rounded-lg bg-muted px-2.5 py-1.5 text-xs placeholder:text-muted-foreground focus:outline-none',
+              showSourceFilter ? 'w-1/2' : 'w-full',
+            )}
             onChange={(e) => {
               setSearch(e.target.value)
               onSearchChange?.(e.target.value)
@@ -216,7 +241,7 @@ function LegacyItemsPanel({
             type="text"
             value={search}
           />
-          {sourceChips.length > 0 && (
+          {showSourceFilter && sourceChips.length > 0 && (
             <div className="flex w-1/2 min-w-0 shrink-0 rounded-lg bg-muted p-0.5">
               {sourceChips.map((chip) => {
                 const isActive = activeSource === chip.id