feat: add keyboard shortcuts and shortcut help modal

[TechSwimmer]

📝 Description

Implemented keyboard shortcuts for key actions in Scikit-Learner to improve usability and workflow efficiency.

Changes Made

• Added keyboard shortcuts for:

  • Train Selected
  • Export Model
  • Upload CSV
  • Shortcut Help Modal (?)

• Added a Shortcut Help Modal to improve shortcut discoverability

• Added a toolbar ? button to open the shortcut help dialog

• Prevented shortcuts from triggering while typing inside inputs and textareas

• Removed Train All shortcut based on maintainer feedback

• Updated shortcut mappings to avoid browser conflicts with reserved shortcuts

🔗 Related Issue

Closes #7

🎯 Type of Change

• fix: Bug fix
• feat: New feature
• style: UI/Theme changes
• chore: Build/Maintenance

✅ Checklist

• Tested keyboard shortcuts locally
• Verified shortcuts do not trigger while typing in input fields
• Added shortcut help modal and toolbar access
• Tested behavior in browser environment
• Kept the PR focused on a single feature

📂 Files Changed

• frontend/js/app.js
• index.html

📸 Screenshots / GIF