Flashcards: reuse card editing for existing decks and manual creation

[AliBenrami]

Parent

Part of #41.

What to build

Add a reusable flashcard editing flow for minor changes to individual cards and deck details. The same editing experience should support existing generated decks and provide a path toward manual flashcard creation.

Acceptance criteria

• Users can edit the front and back of an existing flashcard.
• Users can edit deck-level details such as deck name and supported metadata.
• Users can add and remove individual cards while editing a deck.
• The editor can be used for manual card creation without depending on note-based generation.
• Edits are saved clearly, with cancel/error states that do not silently discard user changes.

Blocked by

• Flashcards: split deck management into a focused My Flashcards page #42
• Flashcards: add generated deck preview and edit-before-save step #44