feedback5 R6: add a correctness-only eslint

Flat config for Node and the browser, with unused names as warnings. The run is clean of errors after three small fixes: the lobby spectate path imports ERR, a disable comment for a rule that is not installed is gone, and two atlas regexes spell the BOM as an escape.

package.json

@@ -34,7 +34,8 @@
     "postinstall": "node tools/vendor.mjs",
     "setup": "node tools/setup.mjs",
     "doctor": "node tools/doctor.mjs",
-    "launch": "node scripts/launch.mjs"
+    "launch": "node scripts/launch.mjs",
+    "lint": "eslint ."
   },
   "engines": {
     "node": ">=22"
@@ -48,6 +49,9 @@
     "ws": "^8.22.0"
   },
   "devDependencies": {
+    "@eslint/js": "^10.0.1",
+    "eslint": "^10.12.0",
+    "globals": "^17.13.0",
     "puppeteer-core": "^25.12.0"
   }
 }