Flashcards: split deck management into a focused My Flashcards page

[AliBenrami]

Parent

Part of #41.

What to build

Create a focused My Flashcards page for managing existing flashcard decks. The page should show the decks a user has created or generated and keep deck management separate from note selection and generation workflows.

Acceptance criteria

• Users can see a list/grid of their existing flashcard decks with enough detail to identify each deck.
• Users can open a deck for review from the My Flashcards page.
• Users can access edit and delete actions for each deck from this management surface.
• The page provides a clear entry point for creating a new flashcard deck without showing note-selection or generation controls inline.
• Empty, loading, and error states are handled without making the page feel crowded.

Blocked by

None - can start immediately.