Add Wi-Fi commissioning: SoftAP captive portal with IPv6 support

Implements first-boot Wi-Fi setup with no companion app required: a
SoftAP + captive portal (DNS wildcard redirect, DHCP option 114, HTTP
form) lets the user submit Wi-Fi credentials from any browser. On
submit, credentials are saved to NVS and the device switches to STA
mode; on subsequent boots it reconnects automatically via stored
credentials. dns_server.c/.h are vendored from ESP-IDF's official
captive_portal example (Unlicense/CC0).

Requests both link-local and global IPv6 addresses once the STA
connection has an IPv4 address (not on WIFI_EVENT_STA_CONNECTED --
esp_netif_create_ip6_linklocal() reliably fails there because
esp-netif's own handler, which marks the lwIP netif "up", hasn't run
yet due to event-handler registration order). Verified end-to-end on
real ESP32-C6 hardware: SoftAP comes up, credential submission works,
STA reconnects on reboot, and both a link-local and a router-assigned
global IPv6 address are obtained via SLAAC.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sdkconfig.defaults

@@ -8,3 +8,9 @@ CONFIG_PARTITION_TABLE_CUSTOM_FILENAME="partitions.csv"
 # Matches the 4MB layout assumed in partitions.csv (TODO: adjust both
 # together once real hardware's actual flash size is confirmed).
 CONFIG_ESPTOOLPY_FLASHSIZE_4MB=y
+
+# IPv6 is required (not just the ESP-IDF default) -- transport/commissioning
+# requests a link-local IPv6 address once Wi-Fi STA connects. Explicit here
+# so it can't silently regress even though CONFIG_LWIP_IPV6 already
+# defaults to y in this ESP-IDF version.
+CONFIG_LWIP_IPV6=y