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_web/components/layouts.ex

@@ -35,13 +35,13 @@ defmodule BerrypodWeb.Layouts do
 
   def app(assigns) do
     ~H"""
+    <.flash_group flash={@flash} />
+
     <main class="app-main">
       <div class="app-container">
         {render_slot(@inner_block)}
       </div>
     </main>
-
-    <.flash_group flash={@flash} />
     """
   end