Deck: Create Deck UI

**Description:** Add a UI form to create a new deck (title + optional description). Persist created decks to localStorage.

**Acceptance criteria:**

There is a visible form/button to create a deck.
New deck requires a non-empty title.
New deck appears in the decks list immediately after creation.
Deck persists after page refresh (saved in localStorage).

Difficulty: Easy

Start at the existing decks list UI and trace how deck data is currently rendered and stored in the client. Add the title-and-description creation flow, then verify that a new deck appears immediately, rejects an empty title, and remains after a page refresh.

Written by the indexing model from the issue text.