Collapse UX to single Usage view + hold-to-pair + status line

Removes the Bluetooth screen — the device now has just splash + Usage. Keeps
main's responsive Layout intact.

- ui: drop the Bluetooth screen, ui_cycle_screen, and SCREEN_BLUETOOTH. The
  bottom spinner becomes a status line — Connected / Disconnected / Pairing…
  (via ble_has_bonds()) plus the whimsical messages when connected and idle.
  A "To pair / hold the power button / for 3 seconds, then release" hint shows
  while disconnected so the screen isn't empty.
- main: hold-to-pair gesture (pair_tick) — hold PWR ~3s then RELEASE clears
  bonds and re-advertises. Clearing on release (not while held) means holding
  to power off (8s hardware shutdown) leaves the bond intact; disarms at 6s.
  PWR short-press now cycles brightness on the Usage view (animations on splash).
- ble: add ble_has_bonds() for the status line.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

firmware/src/ble.cpp

@@ -236,6 +236,10 @@ void ble_clear_bonds(void) {
     need_advertise = true;
 }
 
+bool ble_has_bonds(void) {
+    return NimBLEDevice::getNumBonds() > 0;
+}
+
 bool ble_has_data(void) {
     return data_ready;
 }