feat: category drill-down menu mode

[HerbHall]

Description

Add an alternative menu mode where the main screen shows only the 5 category tiles (Classic, Puzzles, Action, Strategy, Skills) + Settings. Selecting a category transitions into a game list for that category.

Requirements

• Main menu shows categories as selectable tiles (with icon, name, game count)
• Enter on a category transitions to the game list within that category
• Back/Esc returns to category view
• Transition animation between category and game list views
• Configurable: user can choose between flat list (current) and drill-down mode via settings/CLI flag

Dependencies

• Depends on feat: user configuration system with CLI flags #41 (configuration system) for the menu mode toggle
• Depends on feat: transition effects system with multiple effects #43 (transition effects system) for category->game transitions

Notes

Keep both menu modes. The flat list (current) is the default. Category drill-down is opt-in.