Add OTA firmware update support for NRF52840 and ESP32 platforms

[Copilot]

Implements BLE OTA firmware updates triggered from Home Assistant, supporting both NRF52840 (Nordic DFU) and ESP32 (direct BLE OTA) platforms per OpenDisplay_Firmware#1.

NRF52840: Nordic DFU via bootloader

• ble/nrf_dfu.py — Full Nordic Secure DFU protocol implementation (service 0xFE59): SELECT/CREATE/CRC/EXECUTE opcodes, 4096-byte data objects, init packet validation
• Downloads NRF52840.zip (the only asset containing both the .dat init packet and .bin firmware required by the bootloader — .hex/.uf2 lack the init packet)
• Sends cmd 0x0044 → device resets into Adafruit DFU bootloader → scans by service UUID → connects and flashes

ESP32: BLE OTA via application firmware

• ble/esp32_ota.py — Three-command protocol mirroring the direct-write pattern:
  • 0x0046 OTA Start (4-byte LE size) → 0x0047 OTA Data (200-byte chunks, ACK per chunk) → 0x0048 OTA End → reboot
• Downloads application-only .bin (excludes _full.bin merged images)

Platform-aware update entity

• update.py — async_install() reads system.ic_type from TLV config to dispatch:
  • ic_type=1 (NRF52840) → _install_nrf52840() → Nordic DFU
  • ic_type=2,3,4 (ESP32-S3/C3/C6) → _install_esp32() → BLE OTA
• _get_firmware_download_url() resolves correct GitHub release asset per IC type
• UpdateEntityFeature.INSTALL | PROGRESS with percentage callback

Other

• protocol_open_display.py — CMD_OTA_START/CMD_OTA_DATA/CMD_OTA_END constants, enter_dfu_mode() method
• ble/__init__.py — Exports perform_esp32_ota, perform_dfu_update, parse_dfu_package
• strings.json — DFU and ESP32 OTA error strings, updated preview feature description

# IC type → asset mapping
_IC_TYPE_ASSET_PREFIXES = {
    IC_TYPE_NRF52840: "NRF52840",    # → NRF52840.zip (DFU package with init packet)
    IC_TYPE_ESP32_S3: "esp32-s3-",   # → esp32-s3-*.bin (app-only, not _full.bin)
    IC_TYPE_ESP32_C3: "esp32-c3-",
    IC_TYPE_ESP32_C6: "esp32-c6-",
}

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.