Replace the ad-hoc menu_level state machine plan with a uniform
App-per-screen model (home_app/menu_app/time_set_app/pair_app/
ipc_bind_app), each child App exiting to its own parent via long-press
CONFIRM. Folds the sensor-type selection screen into pair_app as a new
initial state, and updates item 5's cyclic-navigation notes to match.
Lists boot-flicker backlight timing, menu idle auto-close, submenu
long-press-confirm exit, default language, cyclic menu navigation, and
P3.6 sleep-wake trigger reliability, each with a brief implementation
approach based on current code.
