build(frontend): register $mocks in kit.alias for tsconfig resolution

The vite resolve.alias (added for the client + coverage runs) does not
reach svelte-check, which resolves paths through the generated tsconfig.
Declaring $mocks in kit.alias feeds both the generated tsconfig paths and
the sveltekit() vite plugin, so editor/type-check resolve it too. Part of #560.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/svelte.config.js

@@ -8,6 +8,11 @@ const config = {
 	preprocess: vitePreprocess(),
 	kit: {
 		adapter: adapter(),
+		// $mocks resolves shared browser-test mock bodies (src/__mocks__). Declared here
+		// so svelte-check/tsconfig and both vite configs resolve it. See ADR-012.
+		alias: {
+			$mocks: 'src/__mocks__'
+		},
 		prerender: {
 			entries: ['/hilfe/transkription'],
 			// Disable crawl: by default SvelteKit follows nav links from