feat: add close button to editor panel

Adds a close button in the top-right corner of the editor panel with double arrow icon. The button closes the panel when clicked.

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

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

src/components/editor/EditorPanel.vue

@@ -27,6 +27,18 @@
       </button>
     </nav>
 
+    <button
+      v-if="activeTab.length > 0"
+      type="button"
+      class="close-button"
+      @click="activeTab = ''"
+      title="Fermer le panneau"
+    >
+      <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="currentColor">
+        <path d="M4.83582 12L11.0429 18.2071L12.4571 16.7929L7.66424 12L12.4571 7.20712L11.0429 5.79291L4.83582 12ZM10.4857 12L16.6928 18.2071L18.107 16.7929L13.3141 12L18.107 7.20712L16.6928 5.79291L10.4857 12Z"></path>
+      </svg>
+    </button>
+
     <div class="tab-content" :class="{ open: activeTab.length > 0 }">
       <div v-if="activeTab === 'document'" class="tab-panel">
         <PageSettings />
@@ -74,6 +86,33 @@ nav {
   z-index: 2;
 }
 
+.close-button {
+  position: absolute;
+  top: 1rem;
+  right: 1rem;
+  z-index: 2;
+
+  width: 2rem;
+  height: 2rem;
+  padding: 0.25rem;
+
+  background: transparent;
+  border: none;
+  cursor: pointer;
+
+  color: var(--color-browngray-300);
+  transition: color 0.2s ease;
+
+  &:hover {
+    color: var(--color-browngray-100);
+  }
+
+  svg {
+    width: 100%;
+    height: 100%;
+  }
+}
+
 .tab-content {
   flex: 1;
   overflow: hidden;