fix(builder): mobile-A2 hardening -- 16px inputs, shared sheet/modal chrome, z-scale + portal

- Force font-size:16px !important on Styles-sheet/topbar/Sitesmith inputs
  inside the mobile media query so inline 12px/14px styles stop triggering
  iOS zoom-on-focus.
- Lift sheet-open + Templates/Head Code modal-open state out of private
  useState into a shared MobileChromeContext (EditorShell), so Phase B can
  open/close sheets from outside MobilePanelBar.
- Add an explicit z-index layer scale, portal TemplateModal to
  document.body (was trapped under the tab bar inside .topbar's stacking
  context), align Sitesmith to the same --z-modal layer, and make opening
  a sheet close any open modal. Also fix modal backdrops swallowing tab
  bar taps (mirrors the sheet backdrop's existing tab-bar cutout).
- Drop BottomSheet's incorrect aria-modal; mobile-aware AssetsPanel empty
  state copy.
- Tests: useIsMobile (matchMedia mock incl. legacy fallback + cleanup),
  MobileChromeContext invariants (one sheet open, sheet closes modals),
  MobilePanelBar wiring.

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

craft/src/panels/left/AssetsPanel.tsx

@@ -2,12 +2,14 @@ import React, { useEffect, useRef, useState, useCallback } from 'react';
 import { useAssets } from '../../hooks/useAssets';
 import { clickableProps } from '../../utils/a11y';
 import { copyToClipboard } from '../../utils/clipboard';
+import { useIsMobile } from '../../hooks/useIsMobile';
 
 /** How long the "Delete?" confirm state stays armed before auto-resetting. */
 const DELETE_CONFIRM_TIMEOUT_MS = 4000;
 
 export const AssetsPanel: React.FC = () => {
   const { assets, loading, error, loadAssets, uploadAsset, deleteAsset } = useAssets();
+  const isMobile = useIsMobile();
   const fileInputRef = useRef<HTMLInputElement>(null);
   const [isDragOver, setIsDragOver] = useState(false);
   const [copiedUrl, setCopiedUrl] = useState<string | null>(null);
@@ -139,7 +141,9 @@ export const AssetsPanel: React.FC = () => {
         {assets.length === 0 && (
           <i className="fa fa-cloud-upload" aria-hidden style={{ fontSize: 28, opacity: 0.5 }} />
         )}
-        {assets.length === 0 ? 'Drag images here or click to upload' : 'Drop files here to upload'}
+        {assets.length === 0
+          ? (isMobile ? 'Tap to upload images' : 'Drag images here or click to upload')
+          : 'Drop files here to upload'}
       </div>
 
       {/* Error message */}