test(i18n): add unit tests for locale detection + extract to module

Extract detectLocale() from hooks.server.ts into src/lib/server/locale.ts
so it can be tested in isolation. Add 7 unit tests covering:
- German, English, Spanish browser preferences
- Fallback when primary language is unsupported
- Quality value (q=) ordering
- Fully unsupported language → null
- Empty Accept-Language header → null

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/hooks.server.ts

@@ -2,24 +2,11 @@ import { redirect, type Handle, type HandleFetch } from '@sveltejs/kit';
 import { paraglideMiddleware } from '$lib/paraglide/server';
 import { sequence } from '@sveltejs/kit/hooks';
 import { env } from 'process';
-import { cookieName, cookieMaxAge, locales } from '$lib/paraglide/runtime';
+import { cookieName, cookieMaxAge } from '$lib/paraglide/runtime';
+import { detectLocale } from '$lib/server/locale';
 
 const PUBLIC_PATHS = ['/login', '/logout'];
 
-function detectLocale(acceptLanguage: string): string | null {
-	const preferred = acceptLanguage
-		.split(',')
-		.map((part) => {
-			const [lang, q] = part.trim().split(';q=');
-			return { lang: lang.trim().split('-')[0].toLowerCase(), q: q ? parseFloat(q) : 1 };
-		})
-		.sort((a, b) => b.q - a.q);
-	for (const { lang } of preferred) {
-		if ((locales as readonly string[]).includes(lang)) return lang;
-	}
-	return null;
-}
-
 const handleLocaleDetection: Handle = ({ event, resolve }) => {
 	if (!event.cookies.get(cookieName)) {
 		const locale = detectLocale(event.request.headers.get('accept-language') ?? '');