Initial commit: ZNET Temperature Sensor ESP32 firmware

- PlatformIO project for Meshnology ESP32 LoRa V3
- DS18B20 dual sensor support for tank verification
- OLED display for local temperature visibility
- HTTP POST to ZNET Web API
- WiFiManager for easy WiFi configuration
- Offline buffering when network unavailable

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,29 @@
+# PlatformIO
+.pio/
+.pioenvs/
+.piolibdeps/
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# Compiled files
+*.o
+*.a
+*.elf
+*.bin
+*.hex
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Build artifacts
+build/
+
+# Credentials (if stored locally)
+credentials.h
+secrets.h