Rename internal URL scheme from browser:// to nebula://

Replaces all occurrences of the 'browser://' internal URL scheme with 'nebula://' across main process, plugin manager, and renderer files. This change standardizes the internal page scheme to 'nebula://' for improved clarity and branding.

plugin-manager.js

@@ -130,7 +130,7 @@ class PluginManager {
       contributeContextMenu: (contribFn) => {
         try { manager._contextMenuContribs.push(contribFn); } catch (e) { console.error(logPrefix, 'contributeContextMenu failed', e); }
       },
-      // Register a dedicated internal page (shown via browser://<id>)
+      // Register a dedicated internal page (shown via nebula://<id>)
     registerRendererPage: ({ id, html }) => {
         try {
           if (!id || !html) return;