UI_ShowSetTimePage() cleared and redrew the entire screen on every
500ms blink toggle and on every up/down value change, causing a
full-screen flash instead of just the intended hour/minute digit
blinking. Split out UI_UpdateSetTimeDigit() which only repaints the
32x32px digit region for the field being edited; the full page draw
is now only used on entry. Field-switch (long-press CONFIRM) refreshes
both digit regions to clear any stale blanked-out state.
Splits the ad-hoc menu_level state machine into standalone Apps with a
uniform parent/child exit convention (long-press CONFIRM -> parent):
clock_app renamed to home_app (boot-default), new set_clock_app
extracted from home_app's time-edit mode, new ipc_bind_app extracted
from menu_app's old level-2 screen, and the sensor-type selection step
merged into pair_app as its new initial state. menu_app now only holds
the 4-item top-level list (language toggle inline). Also makes both
menu_app and pair_app's up/down navigation cyclic (wraps at both ends).
