feat(builder): mobile-responsive editor chrome (Phase A)

Makes the Craft.js editor usable on phones (≤768px) without touching desktop
layout/behavior: a useIsMobile() hook gates a bottom tab bar + sheets (hosting
the existing Blocks/Pages/Layers/Assets/Styles panels unchanged) in place of
the side panels, a collapsed TopBar with a "..." overflow menu, 44px touch
targets, 16px inputs, dvh/safe-area-aware sizing, and small copy/overflow
fixes (empty-canvas hint, Templates modal tabs + close button).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/editor/EditorShell.tsx

@@ -1,12 +1,14 @@
-import React, { useState, useCallback } from 'react';
+import React, { useState, useCallback, useEffect, useRef } from 'react';
 import { useEditor } from '@craftjs/core';
 import { TopBar } from '../panels/topbar/TopBar';
 import { LeftPanel } from '../panels/left/LeftPanel';
 import { RightPanel } from '../panels/right/RightPanel';
+import { MobilePanelBar } from '../panels/mobile/MobilePanelBar';
 import { Canvas } from './Canvas';
 import { ContextMenu } from '../panels/context-menu/ContextMenu';
 import { useContextMenu } from '../hooks/useContextMenu';
 import { useKeyboardShortcuts } from '../hooks/useKeyboardShortcuts';
+import { useIsMobile } from '../hooks/useIsMobile';
 import { DeviceMode } from '../types';
 
 const SHOW_GUIDES_STORAGE_KEY = 'craft-show-guides';
@@ -21,7 +23,21 @@ function loadShowGuides(): boolean {
 }
 
 export const EditorShell: React.FC = () => {
+  const isMobile = useIsMobile();
   const [device, setDevice] = useState<DeviceMode>('desktop');
+  // Phase A: default the canvas to the "mobile" preview width the first
+  // time we detect a mobile viewport, so the frame fits without the user
+  // having to reach for the device switcher (now tucked in TopBar's
+  // overflow menu on mobile). Only fires once, and only if the device is
+  // still at its initial default -- it must not fight a device the user
+  // has already picked (e.g. from the overflow menu) on a later re-render.
+  const mobileDeviceAppliedRef = useRef(false);
+  useEffect(() => {
+    if (isMobile && !mobileDeviceAppliedRef.current) {
+      mobileDeviceAppliedRef.current = true;
+      setDevice((current) => (current === 'desktop' ? 'mobile' : current));
+    }
+  }, [isMobile]);
   // Item 10: canvas dashed "guide" outlines toggle -- default ON, persisted
   // so the choice survives a reload. Lifted here (rather than owned by
   // TopBar or Canvas alone) because the toggle button lives in TopBar but
@@ -67,12 +83,13 @@ export const EditorShell: React.FC = () => {
         onToggleGuides={() => setShowGuides(!showGuides)}
       />
       <div className="editor-container">
-        <LeftPanel />
+        {!isMobile && <LeftPanel />}
         <div onContextMenu={handleContextMenu} style={{ flex: 1, display: 'flex', minWidth: 0 }}>
           <Canvas device={device} showGuides={showGuides} />
         </div>
-        <RightPanel />
+        {!isMobile && <RightPanel />}
       </div>
+      {isMobile && <MobilePanelBar />}
       <ContextMenu
         visible={menuState.visible}
         x={menuState.x}