feat(memorize): add interval preview calculation utility

- Add calculatePreviewIntervals for Again/Hard/Good/Easy buttons
- Support NEW, LEARNING, RELEARNING, and REVIEW card types
- Use SM2_CONFIG constants for accurate interval calculation
- All intervals returned in minutes for consistent formatting

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/app/(features)/memorize/interval-preview.ts

@@ -0,0 +1,84 @@
+import { CardType } from "../../../../generated/prisma/enums";
+import { SM2_CONFIG } from "@/modules/card/card-service-dto";
+
+export interface CardPreview {
+  type: CardType;
+  ivl: number;
+  factor: number;
+  left: number;
+}
+
+export interface PreviewIntervals {
+  again: number;
+  hard: number;
+  good: number;
+  easy: number;
+}
+
+function calculateReviewIntervals(ivl: number, factor: number): PreviewIntervals {
+  const MINUTES_PER_DAY = 1440;
+  return {
+    again: Math.max(1, Math.floor(ivl * SM2_CONFIG.NEW_INTERVAL)) * MINUTES_PER_DAY,
+    hard: Math.floor(ivl * SM2_CONFIG.HARD_INTERVAL * SM2_CONFIG.INTERVAL_MODIFIER) * MINUTES_PER_DAY,
+    good: Math.floor(ivl * (factor / 1000) * SM2_CONFIG.INTERVAL_MODIFIER) * MINUTES_PER_DAY,
+    easy: Math.floor(ivl * (factor / 1000) * SM2_CONFIG.EASY_BONUS * SM2_CONFIG.INTERVAL_MODIFIER) * MINUTES_PER_DAY,
+  };
+}
+
+function calculateNewCardIntervals(): PreviewIntervals {
+  const steps = SM2_CONFIG.LEARNING_STEPS;
+  
+  return {
+    again: steps[0],
+    hard: steps.length >= 2 ? (steps[0] + steps[1]) / 2 : steps[0],
+    good: steps.length >= 2 ? steps[1] : SM2_CONFIG.GRADUATING_INTERVAL_GOOD * 1440,
+    easy: SM2_CONFIG.EASY_INTERVAL * 1440,
+  };
+}
+
+function calculateLearningIntervals(left: number): PreviewIntervals {
+  const steps = SM2_CONFIG.LEARNING_STEPS;
+  const stepIndex = Math.floor(left % 1000);
+  
+  const again = steps[0];
+  
+  let hard: number;
+  if (stepIndex === 0 && steps.length >= 2) {
+    hard = (steps[0] + steps[1]) / 2;
+  } else if (stepIndex < steps.length - 1) {
+    hard = steps[stepIndex];
+  } else {
+    hard = SM2_CONFIG.GRADUATING_INTERVAL_GOOD * 1440;
+  }
+  
+  let good: number;
+  if (stepIndex < steps.length - 1) {
+    good = steps[stepIndex + 1];
+  } else {
+    good = SM2_CONFIG.GRADUATING_INTERVAL_GOOD * 1440;
+  }
+  
+  const easy = SM2_CONFIG.GRADUATING_INTERVAL_EASY * 1440;
+  
+  return { again, hard, good, easy };
+}
+
+export function calculatePreviewIntervals(card: CardPreview): PreviewIntervals {
+  switch (card.type) {
+    case CardType.NEW:
+      return calculateNewCardIntervals();
+    case CardType.LEARNING:
+    case CardType.RELEARNING:
+      return calculateLearningIntervals(card.left);
+    case CardType.REVIEW:
+    default:
+      return calculateReviewIntervals(card.ivl, card.factor);
+  }
+}
+
+export function formatPreviewInterval(minutes: number): string {
+  if (minutes < 1) return "<1";
+  if (minutes < 60) return `${Math.round(minutes)}`;
+  if (minutes < 1440) return `${Math.round(minutes / 60)}h`;
+  return `${Math.round(minutes / 1440)}d`;
+}