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>

test/berrypod_web/live/admin/pages_test.exs

@@ -267,6 +267,113 @@ defmodule BerrypodWeb.Admin.PagesTest do
     end
   end
 
+  describe "undo and redo" do
+    setup %{conn: conn, user: user} do
+      %{conn: log_in_user(conn, user)}
+    end
+
+    test "undo reverts the last change", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/pages/home")
+
+      page = Pages.get_page("home")
+      block = Enum.at(page.blocks, 1)
+      render_click(view, "remove_block", %{"id" => block["id"]})
+
+      # Block should be gone
+      refute has_element?(view, ".block-card-name", "Category navigation")
+
+      # Undo
+      render_click(view, "undo")
+
+      # Block should be back
+      assert has_element?(view, ".block-card-name", "Category navigation")
+      assert has_element?(view, "[aria-live='polite']", "Undone")
+    end
+
+    test "redo restores an undone change", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/pages/home")
+
+      page = Pages.get_page("home")
+      block = Enum.at(page.blocks, 1)
+      render_click(view, "remove_block", %{"id" => block["id"]})
+      render_click(view, "undo")
+
+      assert has_element?(view, ".block-card-name", "Category navigation")
+
+      render_click(view, "redo")
+
+      refute has_element?(view, ".block-card-name", "Category navigation")
+      assert has_element?(view, "[aria-live='polite']", "Redone")
+    end
+
+    test "new mutation clears redo stack", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/pages/home")
+
+      page = Pages.get_page("home")
+      hero = Enum.at(page.blocks, 0)
+      cat_nav = Enum.at(page.blocks, 1)
+
+      # Make a change, undo, then make a different change
+      render_click(view, "remove_block", %{"id" => cat_nav["id"]})
+      render_click(view, "undo")
+      render_click(view, "move_down", %{"id" => hero["id"]})
+
+      # Redo should do nothing (stack was cleared)
+      render_click(view, "redo")
+
+      # Hero should still be at position 2 (the move_down result)
+      assert has_element?(view, "[aria-live='polite']", "Hero banner moved to position 2")
+    end
+
+    test "undo all the way back clears dirty flag", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/pages/home")
+
+      page = Pages.get_page("home")
+      hero = Enum.at(page.blocks, 0)
+      render_click(view, "move_down", %{"id" => hero["id"]})
+
+      assert has_element?(view, ".admin-badge-warning", "Unsaved changes")
+
+      render_click(view, "undo")
+
+      refute has_element?(view, ".admin-badge-warning", "Unsaved changes")
+    end
+
+    test "undo when history empty does nothing", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/pages/home")
+
+      # No changes made, undo should be fine
+      render_click(view, "undo")
+
+      # Should still render normally
+      assert has_element?(view, ".block-card-name", "Hero banner")
+    end
+
+    test "undo/redo buttons reflect stack state", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/admin/pages/home")
+
+      # Initially both disabled
+      assert has_element?(view, "button[phx-click='undo'][disabled]")
+      assert has_element?(view, "button[phx-click='redo'][disabled]")
+
+      # Make a change
+      page = Pages.get_page("home")
+      hero = Enum.at(page.blocks, 0)
+      render_click(view, "move_down", %{"id" => hero["id"]})
+
+      # Undo enabled, redo still disabled
+      refute has_element?(view, "button[phx-click='undo'][disabled]")
+      assert has_element?(view, "button[phx-click='redo'][disabled]")
+
+      # Undo
+      render_click(view, "undo")
+
+      # Undo disabled again, redo now enabled
+      assert has_element?(view, "button[phx-click='undo'][disabled]")
+      refute has_element?(view, "button[phx-click='redo'][disabled]")
+    end
+  end
+
   describe "page editor for page-specific pages" do
     setup %{conn: conn, user: user} do
       %{conn: log_in_user(conn, user)}