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>

main/main.cpp

@@ -1,3 +1,4 @@
+#include "commissioning.h"
 #include "esp_log.h"
 
 namespace {
@@ -7,4 +8,14 @@ constexpr const char *TAG = "kvida_os";
 extern "C" void app_main(void)
 {
     ESP_LOGI(TAG, "kvida-os starting");
+
+    // First boot / no stored Wi-Fi credentials -> fall straight into
+    // commissioning (nothing to protect yet, so no physical-interaction
+    // gate needed here). Re-entering commissioning on an already-configured
+    // device should require a physical trigger (e.g. the user button) per
+    // AGENTS.md's security principle -- not wired up yet since `drivers`
+    // doesn't have a real button implementation.
+    if (!kvida::try_stored_credentials()) {
+        kvida::start_commissioning();
+    }
 }