Leitner / Simple SRS Scheduling

**Description:** Implement a simple Leitner-style SRS: cards have a box and marking known/unknown moves card between boxes and updates nextReview (box → interval in days).

**Acceptance criteria:**
Card objects store box and nextReview.
Marking known increases box (up to max), unknown resets box to 1.
nextReview is computed from box and saved to localStorage.

Difficulty: Medium

Start by locating the Card objects, the known/unknown marking entry points, and the localStorage persistence code. Implement box and nextReview state so known cards advance up to the maximum, unknown cards return to box 1, and the review interval is saved and restored from localStorage. Done means all acceptance criteria are met.

Written by the indexing model from the issue text.