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/idf_component.yml

@@ -0,0 +1,8 @@
+dependencies:
+  mdns:
+    version: "*"
+  # Discovered while implementing this that esp-mqtt is no longer bundled
+  # in ESP-IDF core (moved to the component registry too) -- the plan
+  # assumed it shipped with ESP-IDF, corrected here.
+  espressif/mqtt:
+    version: "*"