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/page_editor_hook_test.exs

@@ -215,6 +215,88 @@ defmodule BerrypodWeb.PageEditorHookTest 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, "/?edit=true")
+
+      blocks = Pages.get_page("home").blocks
+      second_id = Enum.at(blocks, 1)["id"]
+
+      view
+      |> element("button[phx-click='editor_move_up'][phx-value-id='#{second_id}']")
+      |> render_click()
+
+      # After move_up, second block is now first
+      first_html = view |> element(".block-card:first-child") |> render()
+      refute first_html =~ "Hero"
+
+      # Undo
+      view |> element("button[phx-click='editor_undo']") |> render_click()
+
+      # Hero should be back as first block
+      restored_first = view |> element(".block-card:first-child") |> render()
+      assert restored_first =~ "Hero"
+    end
+
+    test "redo restores an undone change", %{conn: conn} do
+      {:ok, view, _html} = live(conn, "/?edit=true")
+
+      blocks = Pages.get_page("home").blocks
+      second_id = Enum.at(blocks, 1)["id"]
+
+      view
+      |> element("button[phx-click='editor_move_up'][phx-value-id='#{second_id}']")
+      |> render_click()
+
+      view |> element("button[phx-click='editor_undo']") |> render_click()
+      view |> element("button[phx-click='editor_redo']") |> render_click()
+
+      # After redo, hero should no longer be first
+      first_html = view |> element(".block-card:first-child") |> render()
+      refute first_html =~ "Hero"
+    end
+
+    test "history clears on save", %{conn: conn} do
+      {:ok, view, _html} = live(conn, "/?edit=true")
+
+      blocks = Pages.get_page("home").blocks
+      second_id = Enum.at(blocks, 1)["id"]
+
+      view
+      |> element("button[phx-click='editor_move_up'][phx-value-id='#{second_id}']")
+      |> render_click()
+
+      view |> element("button[phx-click='editor_save']") |> render_click()
+
+      # After save, undo should be disabled
+      assert has_element?(view, "button[phx-click='editor_undo'][disabled]")
+    end
+
+    test "undo/redo buttons reflect stack state", %{conn: conn} do
+      {:ok, view, _html} = live(conn, "/?edit=true")
+
+      # Initially both disabled
+      assert has_element?(view, "button[phx-click='editor_undo'][disabled]")
+      assert has_element?(view, "button[phx-click='editor_redo'][disabled]")
+
+      # Make a change
+      blocks = Pages.get_page("home").blocks
+      second_id = Enum.at(blocks, 1)["id"]
+
+      view
+      |> element("button[phx-click='editor_move_up'][phx-value-id='#{second_id}']")
+      |> render_click()
+
+      # Undo enabled, redo still disabled
+      refute has_element?(view, "button[phx-click='editor_undo'][disabled]")
+      assert has_element?(view, "button[phx-click='editor_redo'][disabled]")
+    end
+  end
+
   describe "content pages (deferred init)" do
     setup %{conn: conn, user: user} do
       %{conn: log_in_user(conn, user)}