Wire WiFi creds via .env + build-time injection, add heartbeat reporting

Adopts the weather-display pattern: .env file parsed by tools/load_env.py
and injected as -D compiler flags. WIFI_NETWORKS uses "SSID:password" format.
ESP32 now sends a heartbeat POST after each display update so the server
can track frame status for the HA integration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

firmware/platformio.ini

@@ -8,6 +8,9 @@ upload_speed = 921600
 board_build.arduino.memory_type = qio_opi
 board_build.psram = enabled
 
+extra_scripts =
+    pre:tools/load_env.py
+
 build_flags =
     -DBOARD_HAS_PSRAM
     -DARDUINO_USB_CDC_ON_BOOT=1