feat: add backslash shortcut to toggle editor panel

Added \ key to toggle the editor panel open/closed:
- Opens to 'document' tab when panel is closed
- Closes panel when it's open
- Updated button tooltips to indicate the keyboard shortcut

Works in all contexts (main document and iframe).

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

src/App.vue

@@ -52,6 +52,14 @@ const handleKeyboardShortcut = (event) => {
     }
   }
 
+  // Backslash key - toggle editor panel
+  if (event.key === '\\') {
+    event.preventDefault();
+    // Toggle: if panel is closed, open to 'document' tab; if open, close it
+    activeTab.value = activeTab.value.length > 0 ? '' : 'document';
+    return;
+  }
+
   // Cmd+S (Mac) or Ctrl+S (Windows/Linux) - save
   if ((event.metaKey || event.ctrlKey) && event.key === 's') {
     event.preventDefault();