add language learner project

Dutch and Korean (explained in English) and English (explained in Dutch)
from A1 to C1, with a Hangul level for Korean. Lessons with notes, words
and sentences generate multiple-choice, typing, sentence building, cloze,
listening and matching exercises. Endless challenges: a seeded daily
challenge, sprint, survival, mixed practice, level tests, spaced-repetition
reviews and generated drills (numbers, clock times, de/het, plurals, Dutch
and Korean conjugation, particles, Hangul reading, irregular verbs).
Progress is stored in localStorage with export/import.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/routes/projects/language-learner/[lang]/+layout.ts

@@ -0,0 +1,10 @@
+import { error } from '@sveltejs/kit';
+import { isLang, loadCourse } from '$lib/language-learner/courses';
+import { loadDrills } from '$lib/language-learner/drills';
+import type { LayoutLoad } from './$types';
+
+export const load: LayoutLoad = async ({ params }) => {
+	if (!isLang(params.lang)) error(404, 'Unknown language');
+	const [course, drills] = await Promise.all([loadCourse(params.lang), loadDrills(params.lang)]);
+	return { course, drills };
+};