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/hooks/useIsMobile.ts

@@ -0,0 +1,72 @@
+import { useEffect, useState } from 'react';
+
+/** Mobile breakpoint for the editor chrome (Phase A). Keep in sync with the
+ *  `@media (max-width: 768px)` block in `src/styles/editor.css` -- both the
+ *  CSS and this hook must agree on where mobile chrome kicks in. */
+export const MOBILE_BREAKPOINT_PX = 768;
+const MOBILE_QUERY = `(max-width: ${MOBILE_BREAKPOINT_PX}px)`;
+
+function computeIsMobile(): boolean {
+  if (typeof window === 'undefined') return false;
+  if (typeof window.matchMedia === 'function') {
+    try {
+      return window.matchMedia(MOBILE_QUERY).matches;
+    } catch {
+      // Fall through to the width check below (e.g. some older/embedded
+      // WebViews expose a matchMedia that throws instead of omitting it).
+    }
+  }
+  return window.innerWidth <= MOBILE_BREAKPOINT_PX;
+}
+
+/**
+ * Reports whether the viewport is at or below the mobile editor breakpoint.
+ * Drives every mobile-only branch introduced in Phase A (bottom tab bar,
+ * collapsed topbar, bottom sheets) -- desktop rendering must stay identical
+ * above the breakpoint, so this is the single source of truth both React
+ * and (via the matching CSS media query) plain CSS use to agree on "mobile".
+ *
+ * Falls back to a `resize` listener + `window.innerWidth` when
+ * `matchMedia` isn't available (e.g. some test/JSDOM environments), so the
+ * hook degrades gracefully rather than throwing.
+ */
+export function useIsMobile(): boolean {
+  const [isMobile, setIsMobile] = useState<boolean>(computeIsMobile);
+
+  useEffect(() => {
+    if (typeof window === 'undefined') return;
+
+    if (typeof window.matchMedia === 'function') {
+      let mql: MediaQueryList | null = null;
+      try {
+        mql = window.matchMedia(MOBILE_QUERY);
+      } catch {
+        mql = null;
+      }
+
+      if (mql) {
+        const handleChange = () => setIsMobile(mql!.matches);
+        handleChange();
+
+        if (typeof mql.addEventListener === 'function') {
+          mql.addEventListener('change', handleChange);
+          return () => mql!.removeEventListener('change', handleChange);
+        }
+
+        // Safari < 14 only supports the deprecated addListener/removeListener pair.
+        const legacyMql = mql as MediaQueryList & {
+          addListener?: (listener: () => void) => void;
+          removeListener?: (listener: () => void) => void;
+        };
+        legacyMql.addListener?.(handleChange);
+        return () => legacyMql.removeListener?.(handleChange);
+      }
+    }
+
+    const handleResize = () => setIsMobile(window.innerWidth <= MOBILE_BREAKPOINT_PX);
+    window.addEventListener('resize', handleResize);
+    return () => window.removeEventListener('resize', handleResize);
+  }, []);
+
+  return isMobile;
+}