feat: Add keyboard shortcut help screen and fuzzy search (M4.1)

[nathanhuh]

Summary

Improve TUI UX with a help screen showing all keyboard shortcuts and upgrade list filtering to fuzzy search.

Details

• Add ? key binding to show a help screen with all available keyboard shortcuts for the current screen
• Upgrade the current exact-match filter (/ key) to fuzzy search on all list views
• Help screen should be context-aware (show different shortcuts depending on current screen)

Checklist

• Add help screen model and view in internal/app/
• Register ? key binding globally to toggle help overlay
• Generate shortcut list dynamically per screen
• Implement fuzzy matching algorithm (or use a library like sahilm/fuzzy)
• Replace exact substring filter with fuzzy search on all list views
• Write tests