Add a dedicated detection thread that runs face detection continuously
on the latest captured frame and publishes results to a shared dict.
The processing/swap thread reads cached detection results instead of
running detection inline, so it never blocks on the 15-30ms detection
cost.
Architecture change: 2 threads → 3 threads
Before: capture → [detect + swap] → display
After: capture → swap (uses cached detections) → display
↘ detect (async, writes to shared cache) ↗
Also replaces the blocking while/ROOT.update() display loop with
ROOT.after()-based scheduling, which avoids Tk event loop re-entrancy
issues and UI freezes.
Closes#1664
In Tk 9.0, Menu.index("end") returns "" instead of raising TclError
on empty menus. CustomTkinter's DropdownMenu._add_menu_commands
doesn't handle this case, causing a crash when creating CTkOptionMenu
widgets (e.g., the camera selector dropdown).
Add a monkey-patch that guards against the empty-string return value.
