[SYS] Migrate M5 boards to M5Unified + M5GFX (modern platform) (#2355)

The per-board libs (M5Stack/M5StickC/M5StickC-Plus/M5Tough) don't build on
Arduino core 3.x, which kept the five M5 BLE envs pinned to espressif32@6.1.0.
Port board support to M5Unified + M5GFX so they run ${com.esp32_platform} like
the rest of the tree.

- environments.ini: 5 M5 envs -> modern platform + m5unified. m5stick-c uses the
  pico32 variant (m5stick_c variant absent upstream) and no_coredump partitions
  (M5Unified overflows min_spiffs).
- boardM5.cpp / config_M5.h: single <M5Unified.h>; M5.Lcd aliases M5.Display;
  unified brightness/sleep/wake.
- main.cpp / mqttDiscovery.cpp: battery telemetry via M5.Power; AXP192-only
  Vin/VBus/temp fields dropped (no portable equivalent).

All five M5 envs build; M5Stack Core hardware-verified.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

platformio.ini

@@ -148,10 +148,10 @@ fastled = fastled/FastLED@3.9.15
 adafruit_neopixel = adafruit/Adafruit NeoPixel@^1.11.0
 onewire = https://github.com/h2zero/OneWire/archive/GPIO-fix.zip
 dallastemperature = DallasTemperature
-m5stickc = M5StickC@0.2.5
-m5stickcp = https://github.com/m5stack/M5StickC-Plus/archive/0.1.1.zip
-m5stack = M5Stack@0.4.6
-m5tough = https://github.com/m5stack/M5Tough/archive/ccf49e1.zip
+; M5Unified + M5GFX drive all M5 boards (Core / Stick C / Stick C Plus / Tough);
+; they build on the modern Arduino core and auto-detect the board at runtime,
+; replacing the per-board M5Stack/M5StickC/M5StickC-Plus/M5Tough libraries.
+m5unified = m5stack/M5Unified@^0.2.5
 smartrc-cc1101-driver-lib = https://github.com/LSatan/SmartRC-CC1101-Driver-Lib/archive/v3.0.2.zip
 stl = https://github.com/mike-matera/ArduinoSTL/archive/7411816.zip
 shtc3 = https://github.com/sparkfun/SparkFun_SHTC3_Arduino_Library/archive/671ef7e.zip