Global status bar: use quadraui StatusBar adapter (like per-window)

[JDonaghy]

Context

Per-window status bars are already fully shared: build_window_status_line() → window_status_line_to_status_bar() → quadraui StatusBar primitive. Both backends consume identically.

The global status bar (bottom of screen, shows mode + file info when per-window status is disabled) still has per-backend rendering (~50 lines each). It uses build_status_line() which returns plain (String, String, Option<Range>) — no quadraui conversion.

What to do

1. Create build_global_status_bar() -> quadraui::StatusBar in render.rs (analogous to window_status_line_to_status_bar())
2. Both backends call it and draw via their existing draw_status_bar() quadraui bridge
3. Remove the per-backend text layout code

Current locations

• TUI: src/tui_main/panels.rs — render_status_line (~50 lines)
• GTK: src/gtk/draw.rs — draw_status_line (~50 lines)
• Shared: src/render.rs — build_status_line() (returns raw strings, no quadraui conversion)

Priority

Low — small duplication, but completes the pattern established by per-window status bars.