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/lib/server/locale.ts

@@ -0,0 +1,20 @@
+import { locales } from '$lib/paraglide/runtime';
+
+/**
+ * Picks the best supported locale from an Accept-Language header value.
+ * Returns null when no supported locale is found.
+ */
+export 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;
+}