Add search, help screen, and navigation improvements

[nicholasgcoles]

Summary

Improve discoverability and navigation for users managing many tabs across projects.

Tasks

• / or Ctrl+F to filter tabs by project name, branch, or directory
• ? key to toggle a help overlay with all keybindings and feature descriptions
• Mouse support for clicking to select and scroll wheel navigation
• Viewport scrolling with indicator for long tab lists
• Jump-to-project shortcut (e.g. g then first letter)
• Minimum terminal width handling — graceful truncation or wrapping