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.

tools/eslint-rules/unescaped-html-literal.test.ts

@@ -4,13 +4,7 @@
 import rule from './unescaped-html-literal.ts';
 import {RuleTester} from 'eslint';
 
-class VitestRuleTester extends RuleTester {
-  static describe = describe;
-  static it = it;
-  static itOnly = it.only;
-}
-
-const ruleTester = new VitestRuleTester();
+const ruleTester = new RuleTester();
 
 ruleTester.run('unescaped-html-literal', rule, {
   valid: [