Flashcards: add generated deck preview and edit-before-save step

[AliBenrami]

Parent

Part of #41.

What to build

Add a preview step after flashcards are generated and before the deck is saved. Users should be able to review generated cards, make small corrections, and save only when the deck is ready.

Acceptance criteria

• After generation completes, users land on a preview/edit step instead of immediately saving an opaque result.
• Users can edit generated flashcard fronts and backs before saving the deck.
• Users can edit basic deck details such as deck name before saving.
• Users can remove unwanted generated cards before saving.
• Saving persists the edited preview as a deck and returns the user to a useful next step, such as the deck detail or review view.
• Cancel/back behavior is clear and does not accidentally create duplicate decks.

Blocked by

• Flashcards: add a dedicated create flow with note context selection #43