separate editor FAB and panel for cleaner animation

Split the editor sheet into two distinct elements:
- .editor-fab: floating action button, always a pill in the corner
- .editor-panel: sliding panel that animates in/out independently

This enables proper CSS keyframe animations (slide-up/down on mobile,
slide-in/out on desktop) with a closing class for exit transitions.
Simplified the JS hook to only handle close behaviour.

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

lib/berrypod_web/page_renderer.ex

@@ -128,9 +128,9 @@ defmodule BerrypodWeb.PageRenderer do
     ~H"""
     <div id="editor-sheet-inner" phx-hook="EditorKeyboard" data-dirty={to_string(@editor_dirty)}>
       <%!-- Page title and undo/redo --%>
-      <div class="editor-sheet-page-header">
-        <h3 class="editor-sheet-page-title">{@page.title}</h3>
-        <div class="editor-sheet-undo-redo">
+      <div class="editor-panel-page-header">
+        <h3 class="editor-panel-page-title">{@page.title}</h3>
+        <div class="editor-panel-undo-redo">
           <button
             phx-click="editor_undo"
             class={[