feat: timeline range selection + area zoom tools

[lukecotter]

📝 PR Overview

Timeline Measurement Range & Area Zoom

New Features

Measure Range (Shift+Drag)

• Measure the duration between any two points on the timeline
• Displays time span in a floating label with zoom icon
• Resizable by dragging left/right edges
• Double-click inside measurement or click zoom icon to zoom to fit
• Escape or click outside to dismiss

Area Zoom (Alt/Option+Drag)

• Select a time range and instantly zoom to fit
• Quick way to focus on specific timeline sections
• Release mouse to apply zoom

🧩 Type of change (check all applicable)

• 🐛 Bug fix - something not working as expected
• ✨ New feature – adds new functionality
• ♻️ Refactor - internal changes with no user impact
• ⚡ Performance Improvement
• 📝 Documentation - README or documentation site changes
• 🔧 Chore - dev tooling, CI, config
• 💥 Breaking change

📷 Screenshots / gifs / video [optional]

Show off your UI changes.

🔗 Related Issues

fixes #
resolves #164
closes #
related #

✅ Tests added?

• 👍 yes
• 🙅 no, not needed
• 🙋 no, I need help

📚 Docs updated?

• 🔖 README.md
• 🔖 CHANGELOG.md
• 📖 help site
• 🙅 not needed

Anything else we need to know? [optional]