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>

src/entryPoints/WebUiRoutes.cpp

@@ -802,7 +802,7 @@ void registerWebUiRoutes(AsyncWebServer &server) {
         JsonObject objEth = obj["ethernet"];
         if(settings.connType == conn_types_t::ethernet || settings.connType == conn_types_t::ethernetpref)
           reboot = true;
-#ifndef CONFIG_IDF_TARGET_ESP32C6
+#if HAS_ETHERNET
         settings.Ethernet.fromJSON(objEth);
         settings.Ethernet.save();
 #endif
@@ -886,8 +886,10 @@ void registerWebUiRoutes(AsyncWebServer &server) {
     JsonObject obj = doc.to<JsonObject>();
     settings.toJSON(obj);
     obj["fwVersion"] = settings.fwVersion.name;
+    // The web UI hides the Ethernet settings when the chip has no MAC.
+    obj["hasEthernet"] = HAS_ETHERNET == 1;
     JsonObject eth = obj.createNestedObject("ethernet");
-#ifndef CONFIG_IDF_TARGET_ESP32C6
+#if HAS_ETHERNET
     settings.Ethernet.toJSON(eth);
 #endif
     JsonObject wifi = obj.createNestedObject("wifi");