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/mobile/BottomSheet.tsx

@@ -44,7 +44,11 @@ export const BottomSheet: React.FC<BottomSheetProps> = ({ open, onClose, title,
         if (e.target === e.currentTarget) onClose();
       }}
     >
-      <div className="mobile-sheet" role="dialog" aria-modal="true" aria-label={title}>
+      {/* Deliberately NOT aria-modal: the tab bar stays reachable/operable
+          while a sheet is open (tapping another tab swaps sheets directly),
+          so the rest of the screen must not be marked inert to assistive
+          tech the way a true modal dialog would be. */}
+      <div className="mobile-sheet" role="dialog" aria-label={title}>
         <div className="mobile-sheet-handle-row">
           <span className="mobile-sheet-handle" aria-hidden="true" />
         </div>