refactor: integrate StylesheetViewer into EditorPanel code tab

Move StylesheetViewer from standalone fixed panel to integrated component
within EditorPanel's "code" tab. Maintains full functionality including:
- Bidirectional sync with Pinia store and PagedJS preview
- Toggle between read/edit modes
- CSS syntax highlighting
- Debounced updates

Changes:
- EditorPanel.vue: Import and render StylesheetViewer in code tab
- EditorPanel.vue: Add flexbox layout for proper height management
- StylesheetViewer.vue: Convert from fixed positioning to flex container
- App.vue: Remove standalone StylesheetViewer component

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/App.vue

@@ -1,7 +1,6 @@
 <script setup>
 import PagedJsWrapper from './components/PagedJsWrapper.vue';
 import EditorPanel from './components/editor/EditorPanel.vue';
-import StylesheetViewer from './components/StylesheetViewer.vue';
 import ElementPopup from './components/ElementPopup.vue';
 import { onMounted, ref, watch } from 'vue';
 import { useStylesheetStore } from './stores/stylesheet';
@@ -42,7 +41,10 @@ const renderPreview = async (shouldReloadFromFile = false) => {
   `;
 
   iframe.onload = () => {
-    iframe.contentDocument.addEventListener('click', elementPopup.value.handleIframeClick);
+    iframe.contentDocument.addEventListener(
+      'click',
+      elementPopup.value.handleIframeClick
+    );
 
     setTimeout(() => {
       const scrollHeight = iframe.contentDocument.documentElement.scrollHeight;
@@ -55,9 +57,12 @@ const renderPreview = async (shouldReloadFromFile = false) => {
   };
 };
 
-watch(() => stylesheetStore.content, () => {
-  renderPreview();
-});
+watch(
+  () => stylesheetStore.content,
+  () => {
+    renderPreview();
+  }
+);
 
 onMounted(() => renderPreview(true));
 </script>
@@ -71,8 +76,6 @@ onMounted(() => renderPreview(true));
 
   <iframe ref="previewFrame" id="preview-frame"></iframe>
 
-  <StylesheetViewer :stylesheet="stylesheetStore.content" />
-
   <ElementPopup ref="elementPopup" :iframeRef="previewFrame" />
 </template>
 
@@ -80,8 +83,8 @@ onMounted(() => renderPreview(true));
 #preview-frame {
   position: fixed;
   top: 0;
-  left: 250px;
-  width: calc(100% - 600px);
+  left: 0;
+  width: 100vw;
   height: 100vh;
   border: none;
 }