chore(frontend): target ES2022 (#39420)

Bump the TypeScript and Vite targets from ES2020 to ES2022. The frontend
already calls ES2022 APIs like `Object.hasOwn` and `Array.prototype.at`
without polyfills, so downleveling class syntax only added helpers. JS
output shrinks by 36 KB.

The only new syntax-level requirement is Safari 16.4 for mermaid, which
ships class static blocks. Pale Moon supports all emitted syntax since
32.2.

The ES2022 target implies `useDefineForClassFields`, so
`noImplicitOverride` is enabled to flag class fields that shadow
inherited DOM properties.

web_src/js/webcomponents/relative-time.ts

@@ -319,11 +319,11 @@ class RelativeTime extends HTMLElement {
     return undefined;
   }
 
-  get prefix(): string {
+  override get prefix(): string {
     return this.getAttribute('prefix') ?? (this.format === 'datetime' ? '' : 'on');
   }
 
-  set prefix(v: string) {
+  override set prefix(v: string) {
     this.setAttribute('prefix', v);
   }