TUI: Add help screen with keybinding legend

[mihaelamj]

Summary

Add a contextual help screen accessible via ? key showing all keyboard shortcuts.

Design

┌──────────────────────────────────────────────────────────────────────┐
│                        Cupertino TUI - Help                          │
├──────────────────────────────────────────────────────────────────────┤
│ Navigation:                                                          │
│   ↑↓ / j/k    Move cursor up/down                                   │
│   PgUp/PgDn   Jump by page                                           │
│   h / Esc     Return to home                                         │
│                                                                       │
│ Package Management:                                                   │
│   Space       Toggle package selection                               │
│   w           Save selections                                        │
│   o / Enter   Open package in browser                                │
│                                                                       │
│ General:                                                              │
│   ?           Show this help                                         │
│   q / Ctrl+C  Quit                                                   │
├──────────────────────────────────────────────────────────────────────┤
│ Press any key to close                                               │
└──────────────────────────────────────────────────────────────────────┘

Tasks

• Create HelpView.swift with keybinding legend
• Add ViewMode.help case to AppState
• Handle ? key in all views
• Add "Press ? for help" hint in footers
• Test at different terminal sizes

Files

• Sources/TUI/Views/HelpView.swift (new)
• Sources/TUI/Models/AppState.swift
• Sources/TUI/PackageCurator.swift