feat: add keyboard shortcuts Ctrl+1/2/3 to switch between editor tabs

[devendra-w]

Summary

Adds keyboard shortcuts to quickly switch between the HTML, CSS, and JS editor tabs without using the mouse.

Closes #141

Type of Change

• New feature

Changes Made

• Added Ctrl+1 / Cmd+1 → switch to HTML tab
• Added Ctrl+2 / Cmd+2 → switch to CSS tab
• Added Ctrl+3 / Cmd+3 → switch to JS tab
• Shortcuts are added inside the existing keydown handler alongside Ctrl+K (command palette)

How to Test

1. Run npm install and npm run dev
2. Navigate to http://localhost:3000
3. Press Ctrl+1 — verify HTML tab becomes active
4. Press Ctrl+2 — verify CSS tab becomes active
5. Press Ctrl+3 — verify JS tab becomes active
6. Verify shortcuts work on Mac with Cmd+1/2/3 as well

Checklist

• Linked issue in summary
• Self-reviewed the diff
• Added/updated tests if applicable

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[devendra-w]

Hi @Debmallya-03! Added keyboard shortcuts Ctrl+1/2/3 (Cmd+1/2/3 on Mac) to switch between HTML, CSS, and JS editor tabs. The shortcuts are integrated into the existing keydown handler alongside Ctrl+K. I'm already familiar with the codebase from previous contributions. Ready for review!