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/lib/language-learner/text.ts

@@ -0,0 +1,67 @@
+/** Lowercase, strip punctuation and diacritics-insensitive comparison helpers for answers. */
+
+const PUNCTUATION = /[.,!?¿¡;:"“”„‘’'`()[\]{}…~·-]/g;
+
+export function normalize(value: string): string {
+	return value
+		.normalize('NFC')
+		.toLowerCase()
+		.replace(/[’‘`]/g, "'")
+		.replace(PUNCTUATION, ' ')
+		.replace(/\s+/g, ' ')
+		.trim();
+}
+
+export function stripDiacritics(value: string): string {
+	// Only Latin marks: decomposing Hangul would break Korean answers.
+	return value.replace(/[À-ɏ]/g, (c) => c.normalize('NFD').replace(/[̀-ͯ]/g, ''));
+}
+
+export function distance(a: string, b: string): number {
+	const row = Array.from({ length: b.length + 1 }, (_, i) => i);
+	for (let i = 1; i <= a.length; i++) {
+		let previous = row[0];
+		row[0] = i;
+		for (let j = 1; j <= b.length; j++) {
+			const current = row[j];
+			row[j] = Math.min(row[j] + 1, row[j - 1] + 1, previous + (a[i - 1] === b[j - 1] ? 0 : 1));
+			previous = current;
+		}
+	}
+	return row[b.length];
+}
+
+export type Verdict = 'correct' | 'accent' | 'typo' | 'wrong';
+
+/** Compare a typed answer with every accepted answer, forgiving accents and one-letter typos. */
+export function grade(input: string, accepted: string[]): { verdict: Verdict; match: string } {
+	const given = normalize(input);
+	let best: { verdict: Verdict; match: string } = { verdict: 'wrong', match: accepted[0] };
+	if (!given) return best;
+	for (const answer of accepted) {
+		const expected = normalize(answer);
+		if (given === expected) return { verdict: 'correct', match: answer };
+		if (stripDiacritics(given) === stripDiacritics(expected))
+			best = { verdict: 'accent', match: answer };
+		else if (
+			best.verdict === 'wrong' &&
+			[...expected].length >= 5 &&
+			distance(stripDiacritics(given), stripDiacritics(expected)) === 1
+		)
+			best = { verdict: 'typo', match: answer };
+	}
+	return best;
+}
+
+/** Split a sentence into word tiles, keeping punctuation attached to the word. */
+export function tiles(sentence: string): string[] {
+	return sentence.split(/\s+/).filter(Boolean);
+}
+
+export function sameTiles(a: string[], b: string[]): boolean {
+	return normalize(a.join(' ')) === normalize(b.join(' '));
+}
+
+/** Key for telling multiple-choice options apart; unlike `normalize` it keeps apostrophes (its / it's). */
+export const optionKey = (value: string) =>
+	value.normalize('NFC').trim().toLowerCase().replace(/\s+/g, ' ');