Add LilyGo T-Watch S3 variant (non-GPS, 470 mAh)

Same silicon and the same pin map as the T-Watch S3 Plus, verified against the
T_WATCH-S3 schematic (rev 25-03-24) and LilyGo's utilities.h. The differences
are what is populated:

  - no onboard GNSS. HAS_GPS/ENV_INCLUDE_GPS are left undefined, which drops the
    map screen, the GPS home page and BLDO1 rail control via the gates added in
    the MECK_TWATCH rename.
  - no GPIO button. Schematic sheet 1 wires the side switch SW7 to net PWR_KEY
    -> AXP2101 pin 30 (PWRON), so PIN_USER_BTN is undefined and MECK_PMU_BUTTON
    selects PMUButton, which reads PKEY interrupts over I2C. Short press is
    enter/select; a 6s hold is a hardware power-off the firmware never sees.
  - DLDO1 is the MAX98357A speaker rail (schematic sheet 6, net SPK_VDD), not
    'unused' as LilyGo's doc claims. It stays off, so the amp is unpowered.
  - VBACKUP is enabled at 3300 mV. It charges the MS412FE coin cell (J12) that
    backs the PCF8563's single-VDD supply. Leaving it off drained the cell with
    nothing to replenish it.

New envs: meck_twatch_s3_standalone, meck_twatch_s3_ble.
No existing board is affected: MECK_PMU_BUTTON is defined only here.

examples/companion_radio/ui-new/UITask.cpp

@@ -2259,6 +2259,12 @@ void UITask::loop() {
     } else {
       c = checkDisplayOn(KEY_NEXT);
     }
+#elif defined(MECK_PMU_BUTTON)
+    // T-Watch S3: the only key is the AXP2101 PWRON, so a short press is the
+    // sole button gesture available. It acts as enter/select/confirm (in the
+    // contacts screen, that opens the path editor). A >=6s hold is a hardware
+    // power-off and never reaches here.
+    c = checkDisplayOn(KEY_ENTER);
 #else
     c = checkDisplayOn(KEY_NEXT);
 #endif