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/panels/topbar/TemplateModal.tsx

@@ -253,8 +253,10 @@ export const TemplateModal: React.FC<TemplateModalProps> = ({ open, onClose }) =
           </div>
           <button
             onClick={onClose}
+            className="template-modal-close-btn"
             style={closeButtonStyle}
             title="Close"
+            aria-label="Close"
             onMouseEnter={(e) => { (e.currentTarget as HTMLElement).style.background = 'var(--color-bg-hover)'; (e.currentTarget as HTMLElement).style.color = 'var(--color-text)'; }}
             onMouseLeave={(e) => { (e.currentTarget as HTMLElement).style.background = 'transparent'; (e.currentTarget as HTMLElement).style.color = '#71717a'; }}
           >
@@ -262,11 +264,15 @@ export const TemplateModal: React.FC<TemplateModalProps> = ({ open, onClose }) =
           </button>
         </div>
 
-        {/* Filter Tabs */}
-        <div style={tabBarStyle}>
+        {/* Filter Tabs -- horizontally scrollable (tabBarStyle sets
+            overflowX: 'auto') so the row never wraps/overflows the modal at
+            narrow widths; template-modal-tab-btn below keeps each tab from
+            shrinking below a comfortable touch target on mobile. */}
+        <div className="template-modal-tabbar" style={tabBarStyle}>
           {TABS.map((tab) => (
             <button
               key={tab.value}
+              className="template-modal-tab-btn"
               onClick={() => setActiveTab(tab.value)}
               style={{
                 ...tabStyle,