Rename LILYGO_TWATCH_S3_PLUS to MECK_TWATCH as the watch form-factor gate

The macro was doing double duty: gating the 240x240 touch UI (tile grid, lock
screen, VKB, grey palette) and gating the Plus's onboard GNSS. A non-GPS watch
cannot reuse the first without dragging in the second.

  MECK_TWATCH             watch form factor
  LILYGO_TWATCH_S3_PLUS   Plus-only hardware (GNSS on BLDO1, GPIO0 user button)
  MECK_PMU_BUTTON         user button is the AXP2101 PWRON key, not a GPIO

84 sites renamed across 14 files. The WatchMapScreen include, construction and
its three touch handlers now also require HAS_GPS, as does the Maps tile branch
inside the home tile grid.

Behaviour-preserving: the Plus defines both MECK_TWATCH and HAS_GPS=1, so every
new condition evaluates as before. No other board defines MECK_TWATCH.

src/helpers/ui/LGFXDisplay.cpp

@@ -56,14 +56,14 @@ void LGFXDisplay::setColor(Color c) {
       _color = TFT_WHITE;
       break;
     case RED:
-#if defined(LILYGO_TWATCH_S3_PLUS)
+#if defined(MECK_TWATCH)
       _color = 0x7BEF;   // dark grey -- the watch UI uses a grey/white/black palette
 #else
       _color = TFT_RED;
 #endif
       break;
     case GREEN:
-#if defined(LILYGO_TWATCH_S3_PLUS)
+#if defined(MECK_TWATCH)
       _color = 0xC618;   // light grey
 #else
       _color = TFT_GREEN;
@@ -73,7 +73,7 @@ void LGFXDisplay::setColor(Color c) {
       _color = TFT_BLUE;
       break;
     case YELLOW:
-#if defined(LILYGO_TWATCH_S3_PLUS)
+#if defined(MECK_TWATCH)
       _color = TFT_WHITE;
 #else
       _color = TFT_YELLOW;

src/helpers/ui/LGFXDisplay.h

@@ -28,7 +28,7 @@ public:
   void startFrame(Color bkg = DARK) override;
   void setTextSize(int sz) override;
   void setColor(Color c) override;
-#if defined(LILYGO_TWATCH_S3_PLUS)
+#if defined(MECK_TWATCH)
   // Set an exact RGB565 colour, bypassing the Color enum + watch grey remap.
   // Used by the watch P4-style tile grid for per-tile border/fill colours.
   void setRawColor(uint16_t c) { _color = c; buffer.setTextColor(c); }