feat(companion): clock tools — alarm, countdown timer, stopwatch

Add a Clock Tools screen (Enter on the home Clock page) with three time
utilities, plus the engine that drives them from UITask::loop() so they
work regardless of the current screen / display state:

- Alarm: a single one-shot wake alarm. Persisted in NodePrefs as a local
  time-of-day; UITask schedules it as an ABSOLUTE fire instant recomputed
  from that time, so it survives RTC re-syncs (mesh/app/GPS/CLI all jump
  the clock) — small corrections still fire on time, a jump over the
  target still fires (late, up to 6 h). Disarms after firing. A bell
  icon signals an armed alarm on the clock face and the top status bar.
- Timer: a millis-based countdown (sync-immune), big HH:MM:SS readout,
  rings even when off-screen.
- Stopwatch: millis-based, keeps running in the background.

Numeric fields use the shared framework DigitEditor (digit-by-digit,
min/max enforced). The alarm/timer/ring engine lives in UITask alongside
the locator/live-share engines (no screen-cast for time-critical logic).

E-ink: live readouts refresh coarsely (and on any key); timing is exact
regardless, and the countdown's buzzer fires on time. Rings override mute
and are dismissed by any key (auto-stop after 1 min). Cannot wake from a
full Shutdown (CPU/RAM powered down).

NodePrefs: +alarm_on/alarm_hour/alarm_min (fit existing tail padding,
sizeof unchanged 2488), SCHEMA_SENTINEL 0xC0DE0016 -> 0xC0DE0017,
DataStore read/clamp/write in lockstep.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.h

@@ -89,6 +89,7 @@ class UITask : public AbstractUITask {
   UIScreen* compass_screen = nullptr;
   UIScreen* diag_screen = nullptr;
   UIScreen* repeater_screen = nullptr;
+  UIScreen* clock_tools = nullptr;
   UIScreen* curr = nullptr;
   CayenneLPP _dash_lpp;
   TrailStore _trail;
@@ -126,6 +127,25 @@ class UITask : public AbstractUITask {
   void fireLocator(bool arrived);
   void locatorProximityBeeper();
 
+  // Clock tools engine — owned here (not by ClockToolsScreen) so the one-shot
+  // alarm and the countdown timer fire every loop regardless of the current
+  // screen / display state. ClockToolsScreen is pure UI over this state. The
+  // alarm is scheduled as an ABSOLUTE wall instant, recomputed from the stored
+  // time-of-day, so it survives RTC re-syncs (mesh/app/GPS/CLI all jump the
+  // clock) — small corrections still fire on time, a jump over the target still
+  // fires (late). See evaluateAlarm(). Timer + ring are millis-based.
+  uint32_t _alarm_next_fire = 0;   // unix; 0 = (re)compute lazily once time is valid
+  uint32_t _alarm_check_ms  = 0;   // throttle the wall-clock read to ~2 Hz
+  bool     _timer_running = false;
+  uint32_t _timer_deadline_ms = 0;
+  bool     _ringing = false;
+  uint32_t _ring_until_ms = 0;
+  char     _ring_label[20] = {0};
+  uint32_t computeAlarmNextFire(uint32_t now_wall) const;
+  void     evaluateAlarm();                    // alarm scheduling + fire detection
+  void     fireClockAlert(const char* label);  // wake + alert + melody + start ring
+  void     tickClockTools();                   // driven from loop(): ring + timer + alarm
+
   // Course-over-ground ring — a heading source independent of trail recording.
   // Filled from the same periodic GPS poll regardless of _trail.isActive().
   // Heading = bearing across the window (oldest→newest) once the cumulative
@@ -245,6 +265,24 @@ public:
   void gotoCompassScreen();
   void gotoDiagnosticsScreen();
   void gotoRepeaterScreen();
+  void gotoClockTools();   // Alarm / Timer / Stopwatch (from the home Clock page)
+  // Wake the display for an alarm/timer ring (force an immediate refresh).
+  void wakeForAlarm();
+  // Clear any active alert overlay early (alarm dismiss).
+  void clearAlert() { _alert_expiry = 0; }
+  // Clock tools engine API — ClockToolsScreen drives these; the engine itself
+  // runs in tickClockTools() from loop() so it fires regardless of the screen.
+  void onAlarmChanged() { _alarm_next_fire = 0; }   // re-schedule after an alarm edit
+  void startTimer(uint32_t duration_ms) { _timer_running = true; _timer_deadline_ms = millis() + duration_ms; }
+  void stopTimer() { _timer_running = false; }
+  bool isTimerRunning() const { return _timer_running; }
+  uint32_t timerRemainingMs() const {
+    if (!_timer_running) return 0;
+    uint32_t now = millis();
+    return (now >= _timer_deadline_ms) ? 0 : (_timer_deadline_ms - now);
+  }
+  bool isRinging() const { return _ringing; }
+  void dismissRing() { stopMelody(); _ringing = false; clearAlert(); }
   TrailStore& trail() { return _trail; }
   WaypointStore& waypoints() { return _waypoints; }
   LiveTrackStore& liveTrack() { return _livetrack; }