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/ChatInput.tsx

@@ -10,6 +10,7 @@ export const ChatInput: React.FC<Props> = ({ disabled, placeholder, onSend }) =>
     <div style={{ display: 'flex', gap: 8, padding: '8px 0' }}>
       <textarea value={v} onChange={(e) => setV(e.target.value)} onKeyDown={onKey} rows={2} disabled={disabled}
         placeholder={placeholder || 'Describe what you want...'}
+        className="sitesmith-textarea"
         style={{
           flex: 1, background: disabled ? '#1f1f24' : '#0f0f17', color: '#e5e7eb',
           border: '1px solid #3f3f46', borderRadius: 6, padding: 10, fontSize: 14, resize: 'none',