Add MQTT client + Home Assistant Discovery, verified end-to-end

Adds espressif/mqtt and mdns as the project's first external managed
component dependencies (neither ships with ESP-IDF core in v6.0.2
anymore). Once Wi-Fi connects, the device advertises itself on the LAN
as kvida-xxxxxx.local and serves a persistent settings page there for
MQTT broker host/port and credentials -- the ongoing config channel
beyond the one-time Wi-Fi captive portal.

mDNS auto-discovery of _mqtt._tcp was the original plan, but verified
against a real Home Assistant OS + Mosquitto add-on that it doesn't
reliably answer PTR queries on the LAN (confirmed with a raw mDNS
query from the host, parsed properly with dnspython after an earlier
naive byte-check gave a false positive). Manual host/port entry is
now the primary path; mDNS discovery still runs as a secondary
fallback attempt, retried every 30s, in case it works on other
networks.

On MQTT_EVENT_CONNECTED, publishes retained HA MQTT Discovery for a
diagnostic "connectivity" binary_sensor (grouped under a Kvida/Xylon
device block) plus an online/offline availability topic driven by
MQTT's own LWT. Verified end-to-end on real hardware: the device
appears in Home Assistant with a "Connected" Connectivity sensor.

Also extracts url_decode() (previously local to commissioning.cpp)
and a new device_id() helper into shared headers, since both
commissioning's Wi-Fi form and the new MQTT settings form need them.

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

components/transport/CMakeLists.txt

@@ -1,7 +1,7 @@
 idf_component_register(
-    SRCS "src/commissioning.cpp" "src/dns_server.c"
+    SRCS "src/commissioning.cpp" "src/dns_server.c" "src/mqtt.cpp"
     INCLUDE_DIRS "include"
     PRIV_INCLUDE_DIRS "src"
-    PRIV_REQUIRES esp_wifi esp_netif esp_event esp_http_server esp_timer nvs_flash
+    PRIV_REQUIRES esp_wifi esp_netif esp_event esp_http_server esp_timer nvs_flash espressif__mqtt espressif__mdns
     EMBED_FILES "src/portal.html"
 )