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>

apps/editor/components/scene-loader.tsx

@@ -9,9 +9,12 @@ import {
   type SceneGraph,
   type SidebarTab,
 } from '@pascal-app/editor'
+import { Hammer, Layers } from 'lucide-react'
+import Image from 'next/image'
 import Link from 'next/link'
 import { useRouter } from 'next/navigation'
 import { useCallback, useEffect, useRef, useState } from 'react'
+import { BuildTab } from './build-tab'
 import { CommunityViewerToolbarLeft, CommunityViewerToolbarRight } from './viewer-toolbar'
 
 export interface SceneMeta {
@@ -32,6 +35,33 @@ const SIDEBAR_TABS: (SidebarTab & { component: React.ComponentType })[] = [
     id: 'site',
     label: 'Scene',
     component: () => null, // Built-in SitePanel handles this
+    mobileDefaultSnap: 0.5,
+    mobileIcon: <Layers className="h-5 w-5" />,
+    icon: (
+      <Image
+        alt=""
+        className="h-8 w-8 object-contain"
+        height={32}
+        src="/icons/scene.png"
+        width={32}
+      />
+    ),
+  },
+  {
+    id: 'build',
+    label: 'Build',
+    component: BuildTab,
+    mobileDefaultSnap: 0.5,
+    mobileIcon: <Hammer className="h-5 w-5" />,
+    icon: (
+      <Image
+        alt=""
+        className="h-8 w-8 object-contain"
+        height={32}
+        src="/icons/build.png"
+        width={32}
+      />
+    ),
   },
 ]