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.

ui/autocomplete-compnent.ts

@@ -119,7 +119,9 @@ export default class Autocomplete {
       if (!this.visible) {
         if (!this.hideTimeout) {
           document.body.appendChild(this.container);
-          window.getComputedStyle(this.container).opacity;
+          // Force style recalc so the initial opacity is resolved before
+          // setting it to "1", allowing the CSS transition to play.
+          void window.getComputedStyle(this.container).opacity;
         } else {
           clearTimeout(this.hideTimeout);
           this.hideTimeout = null;