feat(udp): custom outbound messages — a trigger, a template, UDP or a URL

The hard-coded emitters each speak one published format at one fixed moment.
This is the escape hatch, and it exists mainly for antenna switches: they are
driven by a URL, and the band change is the trigger they want.

Four triggers, each carrying its own field set: band change (the RADIO's band,
not the entry form — a switch follows the rig, not what is being typed), QSO
logged, rotator command and callsign lookup. The panel prints the fields the
selected trigger can fill, which is the point of the whole thing: a placeholder
the trigger does not carry renders as nothing, and without the list an operator
writes {freq} on a band change and has no way to learn why the switch never
moved.

Three things the panel cannot show, handled here:

  - Escaping. A URL needs every value percent-encoded; a UDP payload must not
    be touched. The first portable callsign, F4BPO/P, puts a path separator in
    the middle of a query string otherwise — it works on the bench and fails on
    the air.
  - Blocking. An HTTP call to a switch that is unplugged would otherwise sit
    for the operating system's timeout, on the path of a band change. It runs
    in the background with a 3 s limit.
  - Silence. A switch answering 404 after a firmware update fails exactly like
    success looks from here, so the status code is logged, throttled per row.

The rotator trigger hooks the COMMAND rather than the SP/LP buttons, so the
compass and a spot click fire it too, and the path ("SP"/"LP") is passed
through because an azimuth alone cannot say which was taken — 137° is short
path to one station and long to another.

Credentials in a URL are stored as typed. That is the operator's call, on the
grounds that this is LAN gear, and the hint in the panel says so.

changelog.json

@@ -13,7 +13,8 @@
       "Motorized antenna: when the controller settles somewhere other than where it was told, the log says so — that reads as “the antenna stopped responding” otherwise.",
       "UDP: an outbound message now logs that it was sent, and a logged QSO with no outbound ADIF row configured says so once.",
       "UDP: new outbound “WSJT-X logged QSO” — announces each contact on the WSJT-X UDP interface, both messages WSJT-X itself sends, for any logger that listens there rather than for plain-text ADIF (Logger32 among them).",
-      "Appearance: the Sahara theme is lighter, parchment rather than deep sand, with a terracotta accent."
+      "Appearance: the Sahara theme is lighter, parchment rather than deep sand, with a terracotta accent.",
+      "UDP: new outbound “Custom message” — you choose what fires it (band change, QSO logged, rotator command, callsign lookup) and what it says, with fields like {band} or {az}. It leaves as a UDP datagram or an HTTP request, which is how most antenna switches are driven."
     ],
     "fr": [
       "Envoi de spot : le commentaire porte désormais les références de diplôme après le mode — celles que vous avez attribuées (POTA, SOTA, IOTA…), pas le DXCC, la zone et le préfixe que chacun déduit de l’indicatif. Un auto-spot porte VOS références d’activation.",
@@ -26,7 +27,8 @@
       "Antenne motorisée : quand le contrôleur se fixe ailleurs que là où on l’a envoyé, le journal le dit — sans quoi cela ressemble à une antenne qui ne répond plus.",
       "UDP : un message sortant inscrit désormais son envoi dans le journal, et un QSO enregistré sans ligne ADIF sortante le signale une fois.",
       "UDP : nouvelle sortie « QSO enregistré WSJT-X » — annonce chaque contact sur l’interface UDP WSJT-X, les deux messages que WSJT-X émet lui-même, pour tout logger qui écoute là plutôt que l’ADIF en texte brut (Logger32 entre autres).",
-      "Apparence : le thème Sahara s’éclaircit, parchemin plutôt que sable profond, avec un accent terre cuite."
+      "Apparence : le thème Sahara s’éclaircit, parchemin plutôt que sable profond, avec un accent terre cuite.",
+      "UDP : nouvelle sortie « Message personnalisé » — vous choisissez ce qui la déclenche (changement de bande, QSO enregistré, commande de rotor, recherche d’indicatif) et ce qu’elle dit, avec des champs comme {band} ou {az}. Elle part en datagramme UDP ou en requête HTTP, ce qui est la façon dont se pilotent la plupart des commutateurs d’antennes."
     ]
   },
   {