fix(a11y): add keyboard activation to sidebar navigation items

[colswap]

Closes #842

Sidebar navigation items use role="button" with tabIndex={0}, making
them focusable for keyboard users, but no onKeyDown handler was present.
Pressing Enter or Space did nothing.

Changes

• Added onKeyDown handler to navTop sidebar items
• Added onKeyDown handler to navBottom sidebar items
• Both handlers trigger onClick on Enter or Space, matching the existing pattern in ProblemPanel.tsx

Flags

• None

Screenshots or Video

• N/A (keyboard interaction, not visual change)

Related Issues

• Issue Bug(a11y): Sidebar navigation items are not activatable via keyboard (Enter/Space) #842

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname

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[github-actions]

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