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>

lib/berrypod_web/page_renderer.ex

@@ -64,14 +64,37 @@ defmodule BerrypodWeb.PageRenderer do
     <div
       id="page-editor-live"
       class="page-editor-live"
-      phx-hook="DirtyGuard"
+      phx-hook="EditorKeyboard"
       data-dirty={to_string(@editor_dirty)}
+      data-event-prefix="editor_"
       data-sidebar-open={to_string(@editor_sidebar_open)}
     >
       <aside class="page-editor-sidebar" aria-label="Page editor">
         <div class="page-editor-sidebar-header">
           <h2 class="page-editor-sidebar-title">{@page.title}</h2>
           <div class="page-editor-sidebar-actions">
+            <button
+              phx-click="editor_undo"
+              class={[
+                "admin-btn admin-btn-sm admin-btn-ghost",
+                @editor_history == [] && "opacity-30"
+              ]}
+              disabled={@editor_history == []}
+              aria-label="Undo"
+            >
+              <.icon name="hero-arrow-uturn-left" class="size-4" />
+            </button>
+            <button
+              phx-click="editor_redo"
+              class={[
+                "admin-btn admin-btn-sm admin-btn-ghost",
+                @editor_future == [] && "opacity-30"
+              ]}
+              disabled={@editor_future == []}
+              aria-label="Redo"
+            >
+              <.icon name="hero-arrow-uturn-right" class="size-4" />
+            </button>
             <button
               phx-click="editor_save"
               class={[