feat: v2.0 rewrite - ESP-IDF firmware + TankSync PWA

Complete rewrite from Arduino to ESP-IDF for both receiver and transmitter.
Added TankSync PWA cloud dashboard with MQTT, push notifications, and QR linking.

Firmware:
- ESP-IDF v5.4 for both ESP32 DevKit and ESP32-C3 receivers
- LoRa pairing protocol, OTA updates (WiFi + LoRa relay)
- Captive portal with auto-redirect (iOS/Android/Windows)
- MQTT auto-discovery for Home Assistant
- WS2812B status LED, remote config via LoRa downlink

PWA (optional cloud dashboard):
- React + Tailwind Progressive Web App
- Fastify + SQLite + MQTT bridge backend
- Multi-site, multi-tank management
- Push notifications, QR code device linking
- Dark/light themes, historical charts

License split: MIT (firmware), AGPL-3.0 (PWA), CC BY-SA 4.0 (hardware)
Legacy Arduino code preserved in v1.0-arduino tag.

Author: Ravi Singh

.github/workflows/build-firmware.yml

@@ -0,0 +1,85 @@
+name: Build & Release Firmware
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include:
+          - name: receiver
+            path: firmware/receiver
+            target: esp32
+            binary: tanksync_receiver.bin
+          - name: receiver-c3
+            path: firmware/receiver-c3
+            target: esp32c3
+            binary: tanksync_receiver.bin
+          - name: transmitter
+            path: firmware/transmitter
+            target: esp32c3
+            binary: tanksync_transmitter.bin
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build with ESP-IDF
+        uses: espressif/esp-idf-ci-action@v1
+        with:
+          esp_idf_version: v5.4
+          target: ${{ matrix.target }}
+          path: ${{ matrix.path }}
+
+      - name: Rename binary
+        run: |
+          cp ${{ matrix.path }}/build/${{ matrix.binary }} \
+             tanksync-${{ matrix.name }}-${{ github.ref_name }}.bin
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: ${{ matrix.name }}
+          path: tanksync-${{ matrix.name }}-${{ github.ref_name }}.bin
+
+  release:
+    needs: build
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/download-artifact@v4
+
+      - name: Create GitHub Release
+        uses: softprops/action-gh-release@v2
+        with:
+          files: |
+            receiver/tanksync-receiver-${{ github.ref_name }}.bin
+            receiver-c3/tanksync-receiver-c3-${{ github.ref_name }}.bin
+            transmitter/tanksync-transmitter-${{ github.ref_name }}.bin
+          generate_release_notes: true
+          body: |
+            ## Firmware Binaries
+
+            | Binary | Board | Description |
+            |--------|-------|-------------|
+            | `tanksync-receiver-${{ github.ref_name }}.bin` | ESP32 DevKit | Receiver firmware |
+            | `tanksync-receiver-c3-${{ github.ref_name }}.bin` | ESP32-C3 | Receiver firmware (C3 variant) |
+            | `tanksync-transmitter-${{ github.ref_name }}.bin` | ESP32-C3 | Transmitter firmware |
+
+            ### Flashing
+            ```bash
+            # Receiver (ESP32 DevKit)
+            esptool.py --chip esp32 -b 460800 write_flash 0x10000 tanksync-receiver-${{ github.ref_name }}.bin
+
+            # Receiver (ESP32-C3)
+            esptool.py --chip esp32c3 -b 460800 write_flash 0x10000 tanksync-receiver-c3-${{ github.ref_name }}.bin
+
+            # Transmitter (ESP32-C3)
+            esptool.py --chip esp32c3 -b 460800 write_flash 0x10000 tanksync-transmitter-${{ github.ref_name }}.bin
+            ```
+
+            Or use the **OTA update** via the receiver's web UI.

.gitignore

@@ -1,25 +1,52 @@
-# Build files
+# Build artifacts (ESP-IDF)
 **/build/
-*.bin
-*.elf
-*.map
+**/sdkconfig
+**/sdkconfig.old
+**/dependencies.lock
+**/managed_components/
 
-# BUT keep firmware release binaries
-!firmware/**/*.bin
+# Node.js
+node_modules/
+dist/
+*.log
+npm-debug.log*
 
-# IDE files
+# Environment & secrets
+.env
+.env.local
+.env.production
+
+# IDE
 .vscode/
 .idea/
 *.code-workspace
 
-# OS files
+# OS
 .DS_Store
 Thumbs.db
 
-# Claude settings
+# Private / AI tooling
 .claude/
 
-# Temporary files
+# Caches
+.cache/
+__pycache__/
+
+# Legacy / dev directories (not part of release)
+v2-qr-linking/
+Receiver-ESP32-DevKit/
+Receiver-IDF/
+Transmitter-IDF/
+tanksync-pwa/
+
+# Large binaries (use GitHub Releases instead)
+*.bin
+*.elf
+*.map
+*.zip
+*.pdf
+
+# Temporary
 *.tmp
 *.bak
 *~