Extract Engine::handle_context_menu_key(key) from backend key handlers

[JDonaghy]

Problem

Both backends implement context menu keyboard navigation (j/k/Up/Down/Escape/Return) by directly mutating cm.selected with wrapping arithmetic:

• GTK: mod.rs:5428-5483, ~55 lines
• TUI: similar in key dispatch, ~40 lines

Proposed fix

Add Engine::handle_context_menu_key(key: &str) -> bool that handles selection movement, confirm, and dismiss. Backends call it instead of reimplementing navigation.

Context

Identified in Session 370 backend dedup audit.

[JDonaghy]

Closing — GTK context menu key handling is dead code. GTK uses native PopoverMenu widgets, not the engine's ContextMenuState. Filed JDonaghy/quadraui#175 to migrate GTK context menus to the quadraui ContextMenu primitive.