replace admin rail with unified bottom sheet editor

- add editor sheet component anchored bottom (mobile) / right (desktop)
- admin cog moves to header, always visible for admins
- remove Done button from editor header, keep only Save
- add editor_at_defaults tracking to disable Reset when at defaults
- sheet collapses on click outside or Escape, stays in edit mode
- dirty indicator + beforeunload warning for unsaved changes
- keyboard shortcuts: Ctrl+Z undo, Ctrl+Shift+Z redo
- WCAG compliant: aria-expanded, live region, focus management

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/berrypod_web/live/shop/custom_page_test.exs

@@ -120,10 +120,18 @@ defmodule BerrypodWeb.Shop.CustomPageTest do
       :ok
     end
 
-    test "editing works with ?edit=true", %{conn: conn, user: user} do
+    test "editing works with edit toggle", %{conn: conn, user: user} do
       conn = log_in_user(conn, user)
-      {:ok, view, _html} = live(conn, "/editable?edit=true")
-      assert has_element?(view, ".page-editor-sidebar")
+      {:ok, view, _html} = live(conn, "/editable")
+
+      # Editor sheet should be visible for admins
+      assert has_element?(view, ".editor-sheet")
+
+      # Click the edit button in the sheet to enter edit mode
+      view |> element(".editor-sheet-edit-btn") |> render_click()
+
+      # Now the editor sheet content should be visible (sheet state changes to open)
+      assert has_element?(view, ".editor-sheet-content")
     end
   end
 end