🎨 Palette: Flashcard Keyboard Shortcuts & Accessibility Hints

[godie]

This PR introduces keyboard shortcuts and visual accessibility hints to the V2 Flashcard Study interface, significantly improving the user experience for repetitive study tasks.

Key Changes:

1. Keyboard Shortcuts:
   • Space or Enter: Flips the card if it's on the front. If already flipped, it submits a "Bien" (Quality 4) rating.
   • Keys 1, 2, 3, and 4: Directly trigger the corresponding quality ratings (Otra vez, Difícil, Bien, Fácil) when the card is flipped.
   • Input Protection: The shortcuts are automatically disabled when the user is focused on an input or textarea.
2. Visual Hints & Accessibility:
   • Added visible hints in brackets (e.g., [Espacio], [3]) to button labels to aid shortcut discovery.
   • Enhanced accessibility with descriptive title and aria-label attributes (e.g., "Atajo: 1. Calificar como Otra vez").
3. Stability & Quality:
   • Corrected hook ordering (useEffect after useCallback) to prevent initialization errors.
   • Added 3 new unit tests to V2FlashcardStudy.test.jsx verifying all shortcut paths.
   • Successfully passed all 365 existing tests and verified visually via Playwright.

These small but impactful changes align with the "Palette" philosophy of adding delight and accessibility through micro-UX improvements.

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PR created automatically by Jules for task 1633578750280856672 started by @godie

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