Fix syntax coloring, modernize toolbar UI, and clean up CSS

- Fix Monarch tokenizer loading: await initLinter() before editor creation
  so loadScript() doesn't clobber window.define during lazy tokenizer init
- Fix JSX/TSX coloring: use file URIs with proper extensions (.jsx/.tsx)
  so Monaco enables JSX tokenization via the TypeScript language service
- Modernize toolbar: move settings to gear popover, replace text buttons
  with SVG icons, consolidate toggle checkboxes into compact group
- Clean up CSS: remove duplicate toggle classes, dead selectors, orphaned rules

public/js/app.js

@@ -290,8 +290,10 @@ function applyLayout(layout) {
 }
 
 async function init() {
-  // Load Emmet before editors so completion providers are registered
+  // Load ALL CDN scripts before editor creation so window.define
+  // (Monaco's RequireJS) is never clobbered during Monarch tokenizer init
   await initEmmet();
+  await initLinter();
 
   // Register custom Monaco themes before creating editors
   registerCustomThemes();
@@ -312,7 +314,6 @@ async function init() {
   initPerformance();
   initResizer();
   initKeybindings();
-  initLinter();
 
   // Auto-run checkbox
   const autoRunCb = $('#auto-run-checkbox');
@@ -571,6 +572,18 @@ async function init() {
   $('#shortcuts-modal-close').addEventListener('click', () => scModal.classList.add('hidden'));
   scModal.addEventListener('click', (e) => { if (e.target === scModal) scModal.classList.add('hidden'); });
 
+  // Settings popover toggle
+  const settingsPopover = $('#settings-popover');
+  $('#btn-settings').addEventListener('click', (e) => {
+    e.stopPropagation();
+    settingsPopover.classList.toggle('hidden');
+  });
+  document.addEventListener('click', (e) => {
+    if (!settingsPopover.classList.contains('hidden') && !e.target.closest('.settings-popover-wrap')) {
+      settingsPopover.classList.add('hidden');
+    }
+  });
+
   // Load fiddle from URL if present
   loadFromUrl();