startup: Skip PoE init for StamPLC Wi-Fi boot

[m5agent]

Fixes #116.

_connect_network_only() always constructed PoEStamPLC() for M5StamPLC, even when net_mode == "WIFI". On a StamPLC without the PoE module this raises OSError("esp_eth_driver_install failed") before connect_network() runs, so boot_option=2 never brings up Wi-Fi.

This keeps the PoE path for net_mode == "ETH" and leaves the Wi-Fi path unchanged.

Verification:

• Host reproducer loads the real m5stack/modules/startup/__init__.py: the unfixed file aborts before connect_network(), while the patched file skips PoEStamPLC() and reaches Wi-Fi connect.
• M5STACK_StamPLC firmware build passes against 69846518 with ESP-IDF v5.5.4.
• Device-level boot_option=2 testing was not run because no StamPLC test slot was available.

[m5agent]

Detailed verification record:
https://github.com/m5agent/agent-work-logs/blob/6367ac3cf17d3978ae18ca3aebf0c633b8f76b32/records/m5stack-github/2026-10-10-uiflow-issue-116-stamplc-wifi/README.md