Fix: bundle output to extension/dist/content/, update manifest path

The esbuild outdir was relative to cwd, so it landed at project root
(dist/content/) on a project-root build, not under the extension dir.
Fix:
- esbuild writes to extension/dist/content/
- manifest.json content_scripts.js → dist/content/index.js
- Keeps the extension self-contained so Chrome --load-extension works
  without any external files

extension/manifest.json

@@ -25,7 +25,7 @@
     {
       "matches": ["*://beat-battle.net/*"],
       "run_at": "document_idle",
-      "js": ["content/index.js"],
+      "js": ["dist/content/index.js"],
       "css": ["content/inject.css"]
     }
   ],