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/ElementPopup.vue

@@ -0,0 +1,152 @@
+<template>
+  <div
+    v-if="visible"
+    id="element-popup"
+    :style="{ top: position.y + 'px', left: position.x + 'px' }"
+  >
+    <div class="popup-header">
+      <span>{{ selector }}</span>
+      <button class="close-btn" @click="$emit('close')">×</button>
+    </div>
+    <div class="popup-body">
+      <div class="popup-controls">
+        <div class="control" v-if="currentFontSize !== null">
+          <label>font-size</label>
+          <input
+            type="number"
+            step="0.1"
+            :value="currentFontSize"
+            @input="$emit('update:fontSize', parseFloat($event.target.value))"
+          />
+          <span>{{ fontSizeUnit }}</span>
+        </div>
+        <p v-else class="no-styles">Aucun style éditable</p>
+      </div>
+      <div class="popup-css">
+        <pre><code class="hljs language-css" v-html="highlightedCss"></code></pre>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup>
+import { computed } from 'vue';
+import hljs from 'highlight.js/lib/core';
+import css from 'highlight.js/lib/languages/css';
+import 'highlight.js/styles/github.css';
+
+hljs.registerLanguage('css', css);
+
+const props = defineProps({
+  visible: Boolean,
+  position: Object,
+  selector: String,
+  elementCss: String,
+  currentFontSize: Number,
+  fontSizeUnit: String
+});
+
+defineEmits(['close', 'update:fontSize']);
+
+const highlightedCss = computed(() => {
+  if (!props.elementCss) return '<span class="no-css">Aucun style défini</span>';
+  return hljs.highlight(props.elementCss, { language: 'css' }).value;
+});
+</script>
+
+<style scoped>
+#element-popup {
+  position: fixed;
+  background: white;
+  border-radius: 4px;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+  z-index: 10000;
+  min-width: 400px;
+  max-width: 500px;
+}
+
+.popup-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 0.5rem 0.75rem;
+  border-bottom: 1px solid #eee;
+  font-size: 0.875rem;
+  font-weight: bold;
+  background: #f5f5f5;
+}
+
+.close-btn {
+  background: none;
+  border: none;
+  cursor: pointer;
+  font-size: 1.25rem;
+  line-height: 1;
+  padding: 0;
+}
+
+.popup-body {
+  display: flex;
+  gap: 1px;
+  background: #eee;
+}
+
+.popup-controls {
+  flex: 1;
+  padding: 0.75rem;
+  background: white;
+}
+
+.popup-css {
+  flex: 1;
+  padding: 0.75rem;
+  background: #1e1e1e;
+  max-height: 200px;
+  overflow-y: auto;
+}
+
+.popup-css pre {
+  margin: 0;
+  font-size: 0.75rem;
+  line-height: 1.4;
+}
+
+.popup-css code {
+  background: transparent;
+  color: #fff;
+}
+
+.control {
+  margin-bottom: 0.5rem;
+}
+
+.control label {
+  display: block;
+  font-size: 0.75rem;
+  margin-bottom: 0.25rem;
+  color: #666;
+}
+
+.control input {
+  width: 60px;
+  padding: 0.25rem;
+  font-size: 0.875rem;
+}
+
+.control span {
+  font-size: 0.75rem;
+  color: #666;
+  margin-left: 0.25rem;
+}
+
+.no-styles {
+  font-size: 0.75rem;
+  color: #999;
+  margin: 0;
+}
+
+.no-css {
+  color: #666;
+  font-style: italic;
+}
+</style>