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/README.txt

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+Copy this folder into Nebula's extensions directory to install:
+
+  %LOCALAPPDATA%\Nebula\User Data\extensions\hello-nebula\
+
+Then open Settings > Plugins and click Reload Plugins (or restart Nebula).
+
+manifest.json fields:
+  id, name, version, description, enabled
+  content_scripts[].matches  (e.g. "*://*/*", "*://*.youtube.com/*", "<all_urls>")
+  content_scripts[].js / css (paths relative to this folder)

examples/extensions/hello-nebula/content.css

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+#nebula-hello-badge {
+  position: fixed;
+  right: 16px;
+  bottom: 16px;
+  z-index: 2147483646;
+  padding: 10px 14px;
+  border-radius: 10px;
+  background: #101820;
+  color: #e8f7ff;
+  font: 600 13px/1.3 Segoe UI, sans-serif;
+  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.28);
+  opacity: 1;
+  transition: opacity 0.4s ease, transform 0.4s ease;
+  pointer-events: none;
+}
+
+#nebula-hello-badge.nebula-hello-badge--hide {
+  opacity: 0;
+  transform: translateY(8px);
+}

examples/extensions/hello-nebula/content.js

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+(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);
+})();

examples/extensions/hello-nebula/manifest.json

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+{
+  "id": "hello-nebula",
+  "name": "Hello Nebula",
+  "version": "1.0.0",
+  "description": "Sample Nebula extension. Shows a small badge on every page.",
+  "enabled": true,
+  "content_scripts": [
+    {
+      "matches": ["*://*/*"],
+      "js": ["content.js"],
+      "css": ["content.css"],
+      "run_at": "document_end"
+    }
+  ]
+}