build: migrate eslint to v10 flat config and upgrade typescript

Replace .eslintrc.json with eslint.config.js using the new flat config
format required by ESLint 10. Swap @typescript-eslint/eslint-plugin and
@typescript-eslint/parser for the unified typescript-eslint package.
Upgrade TypeScript to 5.9 and @types/node to 25. Apply automatic code
fixes: remove unused catch bindings and preserve error cause in auth
header parsing.

package.json

@@ -48,26 +48,27 @@
     "@codemirror/language": "^6.12.2",
     "@codemirror/state": "^6.5.4",
     "@codemirror/view": "^6.39.16",
+    "@eslint/js": "^10.0.1",
     "@tailwindcss/cli": "^4.2.1",
     "@tailwindcss/forms": "^0.5.11",
     "@types/jsonwebtoken": "^9.0.10",
     "@types/koa": "^2.15.0",
     "@types/koa-compress": "^4.0.7",
     "@types/mithril": "^2.2.7",
-    "@types/node": "^20.19.37",
+    "@types/node": "^25.3.5",
     "@types/parsimmon": "^1.10.9",
     "@types/seedrandom": "^3.0.8",
-    "@typescript-eslint/eslint-plugin": "^7.18.0",
-    "@typescript-eslint/parser": "^7.18.0",
     "esbuild": "^0.27.4",
-    "eslint": "^8.57.1",
-    "eslint-config-prettier": "^9.1.2",
+    "eslint": "^10.0.3",
+    "eslint-config-prettier": "^10.1.8",
+    "globals": "^17.4.0",
     "mithril": "^2.3.8",
     "prettier": "^3.8.1",
     "sql.js": "^1.14.1",
     "svgo": "^3.3.3",
     "tailwindcss": "^4.2.1",
     "typescript": "^5.9.3",
+    "typescript-eslint": "^8.56.1",
     "yaml": "^1.10.2"
   },
   "engines": {