Flashcards: add a dedicated create flow with note context selection

[AliBenrami]

Parent

Part of #41.

What to build

Add a dedicated Create Flashcards flow that lets users start a new flashcard deck and select which notes should be used as generation context. This flow should be separate from the My Flashcards management page so reviewing existing decks and creating new decks do not compete for space.

Acceptance criteria

• Users can start a new flashcard deck from the flashcard area.
• Users can select one or more notes to use as context for flashcard generation.
• The selected note context is clearly visible before generation starts.
• Users can change or clear selected notes before generating.
• The create flow has clear loading, empty, and error states for note loading and generation startup.

Blocked by

None - can start immediately.