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/admin.html.heex

@@ -18,9 +18,10 @@
       </.link>
     </header>
 
+    <.flash_group flash={@flash} />
+
     <%!-- page content --%>
     <main class="admin-main">
-      <.flash_group flash={@flash} />
       <div class="admin-container">
         {@inner_content}
       </div>