Add hello-nebula and YouTube-dislike extensions

Add two Nebula extension packages:

- hello-nebula: a minimal demo extension that shows a small badge on pages (manifest, content.js, content.css, README).
- return-youtube-dislike: a content-script port that shows estimated YouTube dislike counts using the public Return YouTube Dislike API (manifest, content.js, content.css, README). Includes GPL-3.0 LICENSE and required attribution. READMEs contain install instructions and notes.

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)

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);
+}

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);
+})();

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"
+    }
+  ]
+}