feat(relays): generic HTTP relay board, and point band switching at it

An operator was about to build band→relay mapping into the custom URL rows.
It already exists, and better: relay auto-control has a per-relay "band" rule
driving webswitch, KMTronic, Dingtian, Denkovi and USB boards. It holds state
so a relay is only commanded when its wanted position changed, reads the
boards live before the first apply so it does not re-command one already in
place, and carries per-relay labels. A URL fired on a band change has none of
that, and would have been a second definition of "which antenna on which band"
— the drift that cost us a week on counties.

So the gap was only the hardware: a hand-made switch is none of the five named
types. It is now the sixth, "HTTP relay": an ON URL and an OFF URL, either as
one pattern with {relay} substituted, or one pair per relay. The per-relay
form is the reason it exists — these boxes often have URLs with nothing in
common between channels (…/FF0101 and …/FF0201), which no pattern can express.

Status is remembered rather than read: most have no endpoint worth trusting.
The cost is stated in the code and the panel — after a restart every relay is
re-commanded once, which is harmless on a board with no memory and far better
than assuming an antenna is already selected.

And the Connections panel now says so, exactly where the wrong choice is made:
picking a band change with a URL transport shows a note pointing at Station
Control. The URL transport stays — a lookup pushed to a webhook or a QSO to a
dashboard is an event, not a state, and Station Control has no place for it.

frontend/src/components/UDPIntegrationsPanel.tsx

@@ -501,6 +501,17 @@ function CustomFields({ draft, setDraft }: {
         </div>
       </div>
 
+      {/* The one place these two features overlap. A relay board is a STATE per
+          band, not an event: Station Control tracks it, reads the boards at
+          startup and does not re-switch while the VFO moves inside a band. A
+          URL fired on a band change has none of that, and building the mapping
+          by hand here is work that is already done there. */}
+      {draft.trigger === 'band_change' && isURL && (
+        <div className="rounded border border-warning-border bg-warning-muted px-2 py-1.5 text-[11px] text-warning-muted-foreground leading-relaxed">
+          {t('udpp.relayInstead')}
+        </div>
+      )}
+
       {isURL ? (
         <div className="space-y-1">
           <Label>{t('udpp.url')}</Label>