Add keyboard navigation and accessible insertion for block palette (a11y)

[Dev10-sys]

Summary

This PR introduces keyboard navigation support for the Block Palette to improve accessibility (a11y) and reduce reliance on mouse-only interactions.

Users can now navigate blocks using arrow keys and insert them into the workspace using the keyboard.

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What’s Added

1. Focus Management

• All palette blocks are focusable via tabIndex={0}
• Clear focus order based on rendered category grouping

2. Keyboard Controls

• ↑ / ↓ → Navigate between blocks in the palette
• Enter / Space → Insert selected block into workspace (center position)
• Escape → Remove focus

3. Workspace Integration

• Added a Recoil atom (keyboardInsertStateAtom) for controlled communication between palette and workspace
• Workspace listens for keyboard-triggered insertions and reuses existing brick factory logic
• No changes to drag-and-drop behavior

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What Was NOT Changed

• Drag-and-drop behavior remains untouched
• Pointer-based brick movement remains unchanged
• Category grouping and search logic unchanged
• No architectural refactoring

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Implementation Notes

• Insertion logic is reused via existing brick factory methods
• Keyboard insertion defaults to center of visible workspace
• Atom state resets after insertion to avoid stale triggers
• No global event listeners were introduced

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Why This Matters

Improves accessibility for:

• Keyboard-only users
• Users with motor impairments
• Screen reader workflows

Aligns with Sugar Labs accessibility goals.

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Testing

• Verified arrow navigation across filtered and grouped bricks
• Verified Enter/Space inserts correct block type
• Verified drag-and-drop still works
• Verified no regressions in workspace behavior

Fixes #498