feat(renderer): AppHeader with sidebar/preview toggles + theme toggle

src/renderer/test/setup.ts

@@ -10,3 +10,20 @@ declare global {
 if (typeof window !== 'undefined' && !window.electronAPI) {
   window.electronAPI = {};
 }
+
+// Mock matchMedia for next-themes (jsdom doesn't have it)
+if (typeof window !== 'undefined') {
+  Object.defineProperty(window, 'matchMedia', {
+    writable: true,
+    value: (query: string) => ({
+      matches: false,
+      media: query,
+      onchange: null,
+      addListener: () => {},
+      removeListener: () => {},
+      addEventListener: () => {},
+      removeEventListener: () => {},
+      dispatchEvent: () => true,
+    }),
+  });
+}