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.

main.js

@@ -1630,7 +1630,7 @@ ipcMain.handle('plugins-get-renderer-preloads', () => {
   try { return pluginManager.getRendererPreloads(); } catch { return []; }
 });
 
-// Plugins: expose registered internal pages (browser://<id>)
+// Plugins: expose registered internal pages (nebula://<id>)
 ipcMain.handle('plugins-get-pages', () => {
   try { return pluginManager.getRendererPages(); } catch { return []; }
 });