feat: iframe-based PagedJS preview with reactive CSS editor

- Isolate PagedJS in iframe to avoid DOM/CSS conflicts
- Add EditorPanel for global CSS controls
- Add StylesheetViewer with highlight.js syntax highlighting
- Add ElementPopup for element-specific CSS editing
- CSS modifications update preview reactively
- Support px/rem/em units

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

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

src/components/SidePanel.vue

@@ -0,0 +1,22 @@
+<template>
+  <aside id="side-panel">
+    <button @click="fontSize++">Action ({{ fontSize }}px)</button>
+  </aside>
+</template>
+
+<script setup>
+const fontSize = defineModel('fontSize');
+</script>
+
+<style>
+#side-panel {
+  position: fixed;
+  top: 0;
+  right: 0;
+  width: 300px;
+  height: 100vh;
+  background: white;
+  box-shadow: -2px 0 8px rgba(0, 0, 0, 0.1);
+  padding: 1rem;
+}
+</style>