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/content.css

@@ -0,0 +1,20 @@
+#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);
+}