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/srs.ts

@@ -0,0 +1,76 @@
+/** A simplified SM-2 schedule. Days are counted as local calendar days. */
+
+export type Card = {
+	due: number;
+	interval: number;
+	ease: number;
+	reps: number;
+	lapses: number;
+	seen: number;
+	right: number;
+	last: number;
+};
+
+export function dayKey(date = new Date()): string {
+	const y = date.getFullYear();
+	const m = String(date.getMonth() + 1).padStart(2, '0');
+	const d = String(date.getDate()).padStart(2, '0');
+	return `${y}-${m}-${d}`;
+}
+
+export function dayNumber(key = dayKey()): number {
+	const [y, m, d] = key.split('-').map(Number);
+	return Math.round(Date.UTC(y, m - 1, d) / 86_400_000);
+}
+
+export const newCard = (today: number): Card => ({
+	due: today,
+	interval: 0,
+	ease: 2.5,
+	reps: 0,
+	lapses: 0,
+	seen: 0,
+	right: 0,
+	last: -1
+});
+
+export function review(card: Card, correct: boolean, today: number): Card {
+	const next = { ...card, seen: card.seen + 1, right: card.right + (correct ? 1 : 0) };
+	if (!correct) {
+		return {
+			...next,
+			reps: 0,
+			interval: 0,
+			lapses: card.lapses + 1,
+			ease: Math.max(1.3, card.ease - 0.2),
+			due: today,
+			last: today
+		};
+	}
+	// Grinding the same card again on one day should not push it months ahead.
+	if (card.last === today && card.reps > 0) return next;
+	const reps = card.reps + 1;
+	const interval =
+		reps === 1 ? 1 : reps === 2 ? 3 : Math.round(Math.max(card.interval, 1) * card.ease);
+	return {
+		...next,
+		reps,
+		interval,
+		ease: Math.min(3, card.ease + 0.05),
+		due: today + interval,
+		last: today
+	};
+}
+
+export type Mastery = 'new' | 'learning' | 'familiar' | 'known' | 'mastered';
+
+export function mastery(card: Card | undefined): Mastery {
+	if (!card || card.seen === 0) return 'new';
+	if (card.interval < 3) return 'learning';
+	if (card.interval < 14) return 'familiar';
+	if (card.interval < 60) return 'known';
+	return 'mastered';
+}
+
+/** 0 for new cards, approaching 1 for cards reviewed months apart. */
+export const strength = (card: Card | undefined) => (card ? Math.min(1, card.interval / 30) : 0);