feat: keep Ethernet settings independent of the framework

Prepares the move to Arduino 3.x / ESP-IDF 5.5, which renumbers
eth_phy_type_t (ETH_PHY_GENERIC is now 0) and only has the RMII MAC
API on the original ESP32.

- Store the PHY chip as our own id (eth_phy_id_t, the 2.x values) and
  translate it by name at ETH.begin(), so NVS, backups and the web UI
  keep their numbers on either framework.
- Build Ethernet only where CONFIG_ETH_USE_ESP32_EMAC is set instead of
  everywhere but the C6.  C3 and S3 could never use it.  A stored
  Ethernet connection falls back to Wi-Fi on those chips.
- /networksettings reports hasEthernet; the UI hides the Ethernet
  options without it.
- Fix the UI PHY list: 5 is the KSZ8041, add the KSZ8081 (6), drop the
  DM9051, an SPI chip the MAC can't drive.
- SSDP takes its address from WiFi/ETH.localIP() instead of the
  tcpip_adapter API IDF 5 removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/stubs/ETH.h

@@ -0,0 +1,16 @@
+// Stand-in for the Arduino core's ETH.h in the native test env.  Uses the
+// Arduino 3.x numbering, which put ETH_PHY_GENERIC first, so a stored PHY id
+// that is cast instead of translated fails the tests.
+#ifndef eth_stub_h
+#define eth_stub_h
+typedef enum {
+  ETH_PHY_GENERIC,
+  ETH_PHY_LAN8720,
+  ETH_PHY_TLK110,
+  ETH_PHY_RTL8201,
+  ETH_PHY_DP83848,
+  ETH_PHY_KSZ8041,
+  ETH_PHY_KSZ8081,
+  ETH_PHY_MAX
+} eth_phy_type_t;
+#endif