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>

lib/berrypod/pages/defaults.ex

@@ -16,6 +16,20 @@ defmodule Berrypod.Pages.Defaults do
     |> Enum.map(&for_slug/1)
   end
 
+  @doc "Returns true if the given blocks match the defaults for the slug."
+  def matches_defaults?(slug, current_blocks) when is_list(current_blocks) do
+    default_blocks = blocks(slug)
+
+    length(current_blocks) == length(default_blocks) and
+      Enum.zip(current_blocks, default_blocks)
+      |> Enum.all?(fn {current, default} ->
+        current["type"] == default["type"] and
+          current["settings"] == default["settings"]
+      end)
+  end
+
+  def matches_defaults?(_slug, _blocks), do: false
+
   # ── Page titles ─────────────────────────────────────────────────
 
   defp title("home"), do: "Home page"