GTK: replace native PopoverMenu context menus with quadraui ContextMenu primitive

[JDonaghy]

Problem

GTK backend uses native gtk4::PopoverMenu for explorer, tab, and editor right-click context menus instead of the quadraui ContextMenu primitive. This means:

• ~200 lines of GTK-specific GIO action wiring in src/gtk/mod.rs (show_explorer_context_menu, handle_tab_right_click, show_action_menu_popover)
• engine.handle_context_menu_key() is dead code on GTK — j/k nav and disabled-item skipping don't work
• Cross-backend coverage table claims ContextMenu ✅/✅ but explorer/tab menus are actually native popovers

TUI already uses the engine ContextMenuState → quadraui ContextMenu path with full keyboard nav.

Current GTK flow

1. Right-click → engine.open_*_context_menu() to build items
2. Clone items, immediately engine.close_context_menu()
3. Build native PopoverMenu with GIO SimpleAction wiring
4. Native popover handles keyboard/mouse

Target flow

1. Right-click → engine.open_*_context_menu() (same)
2. GTK renders via quadraui ContextMenu primitive (quadraui GTK rasteriser already exists)
3. Keyboard via engine.handle_context_menu_key() (same as TUI)

Files to change

• src/gtk/mod.rs: remove show_explorer_context_menu (~100 lines), handle_tab_right_click (~80 lines), native popover wiring; render engine's context_menu through quadraui primitive instead
• src/gtk/mod.rs: remove build_gio_menu_from_engine_items helper if it becomes unused

Context

Discovered during backend dedup audit (Session 373, #379). Quadraui confirms the GTK ContextMenu rasteriser is fully implemented (quadraui#175).

[JDonaghy]

Starting work on this. Branch: issue-395-popovermenu-to-quadraui. Picking up as the next item in the macOS-readiness dedup audit (#417 ✅, #418 → PR #424, #395 ← here, #274 last).