feat: OTA firmware update per device with auto config backup

- Upload .bin firmware files via web UI
- Update individual devices (button per device in table)
- Auto config backup before each update
- Shows current version vs available firmware
- Firmware file management (upload/delete)

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

.gitignore

@@ -8,6 +8,7 @@ config.cfg
 config.dev.cfg
 *.egg-info/
 backups/
+firmware/
 dist/
 build/
 .vscode/