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/sitesmith/SitesmithModal.tsx

@@ -98,7 +98,7 @@ export const SitesmithModal: React.FC<Props> = ({ onClose, target }) => {
       onClose={onClose}
       closeOnEscape={false}
       closeOnBackdropClick={false}
-      backdropStyle={{ backgroundColor: 'rgba(0,0,0,0.8)', zIndex: 9000 }}
+      backdropStyle={{ backgroundColor: 'rgba(0,0,0,0.8)' }}
       backdropProps={{ role: 'dialog', 'aria-modal': true }}
     >
       <div style={panel}>