add undo/redo to page editors with keyboard shortcuts

History stacks (@history/@future) on both admin editor and live sidebar,
capped at 50 entries. All mutations routed through apply_mutation for
consistent history tracking. EditorKeyboard JS hook combines DirtyGuard
with Ctrl+Z/Ctrl+Shift+Z. Settings panel fade-in animation. 10 new tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/js/app.js

@@ -651,10 +651,41 @@ const DirtyGuard = {
   }
 }
 
+// DirtyGuard + Ctrl+Z / Ctrl+Shift+Z undo/redo for page editors
+const EditorKeyboard = {
+  mounted() {
+    this._beforeUnload = (e) => {
+      if (this.el.dataset.dirty === "true") {
+        e.preventDefault()
+        e.returnValue = ""
+      }
+    }
+    window.addEventListener("beforeunload", this._beforeUnload)
+
+    const prefix = this.el.dataset.eventPrefix || ""
+    this._keydown = (e) => {
+      if ((e.ctrlKey || e.metaKey) && e.key === "z") {
+        e.preventDefault()
+        if (e.shiftKey) {
+          this.pushEvent(prefix + "redo")
+        } else {
+          this.pushEvent(prefix + "undo")
+        }
+      }
+    }
+    document.addEventListener("keydown", this._keydown)
+  },
+
+  destroyed() {
+    window.removeEventListener("beforeunload", this._beforeUnload)
+    document.removeEventListener("keydown", this._keydown)
+  }
+}
+
 const csrfToken = document.querySelector("meta[name='csrf-token']").getAttribute("content")
 const liveSocket = new LiveSocket("/live", Socket, {
   params: {_csrf_token: csrfToken, screen_width: window.innerWidth},
-  hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer, ProductImageScroll, SearchModal, CollectionFilters, CardRadioScroll, AnalyticsInit, AnalyticsExport, ChartTooltip, DirtyGuard},
+  hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer, ProductImageScroll, SearchModal, CollectionFilters, CardRadioScroll, AnalyticsInit, AnalyticsExport, ChartTooltip, DirtyGuard, EditorKeyboard},
 })
 
 // Show progress bar on live navigation and form submits