Replace bespoke menu bar nav arrows + search box with quadraui::CommandCenter

[JDonaghy]

Context

#301 (PR #309) migrated the menu bar labels to quadraui::MenuBar. The nav arrows (◀ ▶) and centered search/title box remained as ~242 lines of bespoke per-backend code because the quadraui primitive didn't exist yet.

JDonaghy/quadraui#51 shipped the CommandCenter primitive with TUI + GTK rasterisers and paint↔click round-trip harnesses.

Scope

• Build render::build_command_center_view() adapter from engine state (tab_nav_can_go_back, tab_nav_can_go_forward, title)
• Replace TUI bespoke nav arrows + search box paint in render_menu_bar() (~65 lines) with quadraui::tui::draw_command_center()
• Replace GTK bespoke nav arrows + search box paint in draw_menu_bar() (~119 lines) with quadraui::gtk::draw_command_center()
• Replace TUI bespoke click geometry in mouse.rs (~39 lines) with CommandCenterLayout::hit_test()
• Replace GTK cached nav rects click handling (~19 lines) with CommandCenterLayout::hit_test()
• Cache CommandCenterLayout on engine for click dispatch (same pattern as MenuBarLayout)
• Remove nav_arrow_rects_cell from GTK (superseded by the layout cache)

Where to look

• src/tui_main/render_impl.rs — render_menu_bar() lines after draw_menu_bar() call
• src/gtk/draw.rs — draw_menu_bar() lines after quadraui::gtk::draw_menu_bar() call
• src/tui_main/mouse.rs — nav arrow + search box click handling after MenuBarLayout::hit_test
• src/gtk/mod.rs — nav_rects_click usage in click gesture + nav_arrow_rects_cell declaration

Out of scope

• Menu bar keyboard dispatch gaps (Menu bar keyboard/hover dispatch gaps (GTK arrow keys, TUI Alt conflicts, TUI hover) #308)
• Menu dropdown rendering (already uses quadraui ContextMenu)

[JDonaghy]

Implemented in b5fdd7d (landed Path A on develop)