Implement Keyboard Shortcuts for Tools

[shivam7147]

Description

• Added keyboard shortcut support for tool selection in the canvas.
• Users can now switch between tools (Pen and Eraser) using keyboard keys while focused on the canvas.
• The active tool visually updates in the Toolbar when changed via shortcuts, and shortcuts are only active when the canvas is focused.

Semver Changes

• Patch (bug fix, no new features)
• Minor (new features, no breaking changes)
• Major (breaking changes)

Issues

Closes #10 — Implement keyboard shortcuts for tool selection.

Checklist

• I have read the Contributing Guidelines
• Added keyboard shortcut handling (P/1 for Pen, E/2 for Eraser)
• Ensured shortcuts only work when the canvas is focused
• Updated Toolbar to visually reflect the active tool when changed via shortcuts
• Tested UI and focus behavior manually