Upgrade typedoc-github-theme to v0.4.0 and remove local CSS workaround

v0.4.0 includes the typography fixes from upstream PR
JulianWowra/typedoc-github-theme#7, so the local `customCss` override
(`scripts/typedoc-github-theme-fixes.css`) added in 0bbbfee is no
longer needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -99,7 +99,7 @@
     "ts-to-zod": "^5.1.0",
     "tsx": "^4.21.0",
     "typedoc": "^0.28.14",
-    "typedoc-github-theme": "^0.3.1",
+    "typedoc-github-theme": "^0.4.0",
     "typescript": "^5.9.3",
     "zod": "^4.1.13"
   },