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>

data-src/index.html

@@ -277,7 +277,7 @@
                 <div id="divNetworkSettings" style="display:none;">
                     <div class="subtab-container"><span class="selected" data-grpid="divNetAdapter">Adapter</span><span data-grpid="divDHCP">DHCP/Static IP</span><span data-grpid="divMQTT">MQTT</span></div>
                     <div id="divNetAdapter" class="subtab-content">
-                        <div class="field-group" style="vertical-align:middle;color:#00bcd4;margin-top:-24px;">
+                        <div id="divHardwired" class="field-group" style="vertical-align:middle;color:#00bcd4;margin-top:-24px;">
                             <input id="cbHardwired" name="hardwired" data-bind="ethernet.hardwired" type="checkbox" style="display:inline-block;" onclick="wifi.useEthernetClicked();" />
                             <label for="cbHardwired" style="display:inline-block;cursor:pointer;">Use Ethernet</label>
                             <div id="divFallbackWireless" style="display:inline-block;padding-left:7px;">