fix: bundle the content script to stop 'Unsafe attempt to load URL' errors

Dynamic import() of extension modules from a content script makes Chrome log
'Unsafe attempt to load URL chrome-extension://...' on YouTube pages, which fills
the extension's error list even though the modules load (HTTP 200). The content
script is now one classic file built by 'npm run build'; web_accessible_resources
is no longer needed and is removed.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

package.json

@@ -4,6 +4,7 @@
   "private": true,
   "type": "module",
   "scripts": {
+    "build": "node tools/build-content.js",
     "test": "node --test"
   }
 }