feat(i18n): detect browser language as default locale

On first visit (no PARAGLIDE_LOCALE cookie), parse the Accept-Language
request header and set the cookie to the best matching supported locale
(de/en/es). The user's manual choice via the switcher always takes
precedence since the detection is skipped when the cookie exists.

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

frontend/src/hooks.server.ts

@@ -2,9 +2,34 @@ 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';
 
 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') ?? '');
+		if (locale) {
+			event.cookies.set(cookieName, locale, { path: '/', sameSite: 'lax', maxAge: cookieMaxAge });
+		}
+	}
+	return resolve(event);
+};
+
 const handleAuth: Handle = async ({ event, resolve }) => {
 	const isPublic = PUBLIC_PATHS.some((p) => event.url.pathname.startsWith(p));
 	if (!isPublic && !event.locals.user) {
@@ -74,4 +99,4 @@ export const handleFetch: HandleFetch = async ({ event, request, fetch }) => {
     return fetch(request);
 };
 
-export const handle = sequence(userGroup, handleAuth, handleParaglide);
+export const handle = sequence(userGroup, handleAuth, handleLocaleDetection, handleParaglide);