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 <[email protected]>

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

@@ -0,0 +1,24 @@
+import { error } from '@sveltejs/kit';
+import { marked } from 'marked';
+import { allLessons } from '$lib/language-learner/course';
+import type { PageLoad } from './$types';
+
+export const load: PageLoad = async ({ parent, params }) => {
+	const { course } = await parent();
+	const lessons = allLessons(course);
+	const index = lessons.findIndex((l) => l.id === params.lesson);
+	if (index < 0) error(404, 'Unknown lesson');
+	const lesson = lessons[index];
+	const level = course.levels.find((lv) => lv.id === lesson.level)!;
+	return {
+		lesson,
+		level,
+		next: lessons[index + 1] ?? null,
+		// Notes are course content written in this repository, not user input.
+		notesHtml: marked.parse(lesson.notes, { async: false }),
+		seo: {
+			title: `${lesson.title} · ${course.name} ${level.band}`,
+			description: `${course.name} lesson: ${lesson.summary}. ${lesson.words.length} words with exercises.`
+		}
+	};
+};