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.

bin/genieacs-ext.ts

@@ -55,6 +55,7 @@ process.on("message", (message) => {
   if (!script) {
     const cwd = process.env["GENIEACS_EXT_DIR"];
     process.chdir(cwd);
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
     script = require(`${cwd}/${fileName}`);
   }