Add unpacked extension support

Adds an extension manager that loads unpacked extensions from the user data extensions directory, persists enabled state, and injects matching content scripts on page load. The settings Plugins page now shows installed extensions, supports reload/toggle actions, and includes an example Hello Nebula extension.

examples/extensions/hello-nebula/content.js

@@ -0,0 +1,15 @@
+(function () {
+  if (document.getElementById('nebula-hello-badge')) {
+    return;
+  }
+
+  const badge = document.createElement('div');
+  badge.id = 'nebula-hello-badge';
+  badge.textContent = 'Nebula extension loaded';
+  badge.title = 'hello-nebula sample extension';
+  document.documentElement.appendChild(badge);
+
+  window.setTimeout(() => {
+    badge.classList.add('nebula-hello-badge--hide');
+  }, 3500);
+})();