Due Filter & Session Mode

**Description:** Implement a “Study session” mode which filters and shows only cards whose nextReview timestamp is due (<= now). Session should iterate through due cards only.

**Acceptance criteria:**
A "Study session" mode/button exists and starts a session with only due cards.
If no cards are due, a friendly message is shown.
Session actions (mark known/unknown) update card scheduling and persist.

Difficulty: Medium

Start by locating the card display/session entry point and the code that reads nextReview and persists card scheduling. Trace how mark known/unknown actions update cards, then implement due-card filtering and the no-due-cards message. Done means a session shows only cards with nextReview <= now, handles an empty session clearly, and persists both actions.

Written by the indexing model from the issue text.