Clean up Makefile, tests and legacy code (#36638)

This simplifies the Makefile by removing the whole-file wrapping that
creates a tempdir introduced by
https://github.com/go-gitea/gitea/pull/11126. REPO_TEST_DIR is removed
as well.

Also clean up a lot of legacy code: unnecessary XSS test, incorrect test
env init, unused "_old_uid" hack, etc

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

package.json

@@ -77,6 +77,7 @@
     "@types/jquery": "3.5.33",
     "@types/js-yaml": "4.0.9",
     "@types/katex": "0.16.8",
+    "@types/node": "25.2.3",
     "@types/pdfobject": "2.2.5",
     "@types/sortablejs": "1.15.9",
     "@types/swagger-ui-dist": "3.30.6",
@@ -119,9 +120,6 @@
     "vitest": "4.0.18",
     "vue-tsc": "3.2.4"
   },
-  "browserslist": [
-    "defaults"
-  ],
   "pnpm": {
     "overrides": {
       "array-includes": "npm:@nolyfill/array-includes@^1",