Create a complete embedded time synchronization system using:

- ATtiny1614 (UART + I2C)
- DS1307 RTC with coin cell
- Seeed Studio XIAO ESP32-S3 (Arduino framework)
- Web Bluetooth API web application

System architecture:
Web App → BLE → ESP32-S3 → UART → ATtiny1614 → DS1307 RTC

IMPORTANT PIN CONFIG (must be used exactly):
ESP32:
- TX_PIN = D6
- RX_PIN = D7
- LED_PIN = D9

ATtiny1614:
- PB2 = TX
- PB3 = RX
- PB0 = SCL
- PB1 = SDA

Requirements:

1. ATtiny1614 firmware:
- Communicate with DS1307 over I2C
- Store timezone offset in EEPROM
- Receive UART command:
  S,YYYY,MM,DD,HH,MM,SS,OFFSET
- Treat received time as UTC
- Convert UTC → local time using accurate calendar (leap year, real months)
- Write local time to RTC
- Continuously output:
  Machine format: T,YYYY,MM,DD,HH,MM,SS
  Human format: DATE:YYYY/MM/DD TIME:HH:MM:SS
- Baud rate: 9600

2. ESP32-S3 firmware:
- BLE server (Web Bluetooth compatible)
- Receive JSON:
  { "utc": "...", "offset": seconds, "tz": "string" }
- Parse JSON safely
- Send UART command to ATtiny
- Read UART continuously
- Print:
  - raw UART data
  - time before sync
  - time after sync
  - live time every 5 seconds
- Handle BLE timeout (2 minutes)
- Use HardwareSerial with D6 (TX) and D7 (RX)

3. Web App:
- Runs on mobile Chrome
- Uses Web Bluetooth
- Uses Intl.supportedValuesOf("timeZone") for full timezone list
- Searchable UI (city/timezone)
- Calculates correct offset including DST
- Sends JSON to ESP32

4. Constraints:
- No WiFi on ESP32
- Phone provides accurate time
- System must run offline after sync
- UART must be stable and debuggable

Provide:
- Complete ATtiny code
- Complete ESP32 code
- Complete HTML web app
- UART debug strategy
- Notes on wiring and voltage compatibility