feat(kpa): the Elecraft amplifiers in the Settings, the card and the widget

Wired into the multi-amplifier support that already carries the SPE, the
ACOM and the PowerGenius: a fourth brand rather than a fourth mechanism.
The linking, the fan-out and the poll all work on it unchanged.

Three things the KPA does differently, and each is handled where it shows
rather than explained in a hint:

  - A KPA500 has no network side at all. Choosing it puts the entry on
    serial, and a configuration still asking for TCP is refused with a
    line saying so instead of retrying an address that cannot answer.
  - The amplifier answers ^ON while its main supplies are OFF — a sleeping
    microcontroller stays awake for exactly that — so power-on stays
    available over the network, unlike the SPE and ACOM which need their
    serial control lines.
  - Going to OPERATE clears the current fault, everything except
    temperature. The widget says so under the fault, because the button
    that fixes it is the one already on screen.

Defaults that leave a working configuration when the model is changed:
38400 baud, TCP port 1500, and a full-scale power mark of 500 or 1500 W
by model — a KPA500 read against a 1500 W scale looks idle at full
output.

frontend/src/lib/i18n.tsx

@@ -419,7 +419,7 @@ const en: Dict = {
   'agp.portDeselect': 'Port {letter} — click to deselect', 'agp.portSelect': 'Select on port {letter}', 'agp.online': 'online', 'agp.offline': 'offline', 'agp.close': 'Close', 'agp.connecting': 'Connecting…', 'agp.noAntennas': 'No antennas configured.', 'agp.filterOnHint': 'Showing antennas for {band} only — click to show all bands', 'agp.filterOffHint': 'Showing all antennas — click to show only the current band',
   'ampw.title': 'Amplifier', 'ampw.all': 'All amplifiers', 'ampw.pick': 'Which amplifier the widget shows',
   'ampw.showHint': 'Amplifier · click to show', 'ampw.hideHint': 'Amplifier — shown · click to hide',
-  'ampw.close': 'Close', 'ampw.offline': 'offline', 'ampw.none': 'No amplifier configured',
+  'ampw.close': 'Close', 'ampw.offline': 'offline', 'ampw.kpaClear': 'OPERATE clears this fault', 'ampw.none': 'No amplifier configured',
   'ampw.operate': 'Operate', 'ampw.standby': 'Standby', 'ampw.on': 'ON', 'ampw.off': 'OFF',
   'ampw.lvlL': 'Low', 'ampw.lvlM': 'Mid', 'ampw.lvlH': 'High',
   'ampw.pwr': 'Watts', 'ampw.swr': 'SWR', 'ampw.temp': 'Temp', 'ampw.id': 'Id (A)', 'ampw.fan': 'Fan mode',
@@ -891,7 +891,7 @@ const fr: Dict = {
   'agp.portDeselect': 'Port {letter} — clic pour désélectionner', 'agp.portSelect': 'Sélectionner sur le port {letter}', 'agp.online': 'en ligne', 'agp.offline': 'hors ligne', 'agp.close': 'Fermer', 'agp.connecting': 'Connexion…', 'agp.noAntennas': 'Aucune antenne configurée.', 'agp.filterOnHint': 'Antennes du {band} uniquement — clic pour afficher toutes les bandes', 'agp.filterOffHint': 'Toutes les antennes affichées — clic pour n’afficher que la bande courante',
   'ampw.title': 'Amplificateur', 'ampw.all': 'Tous les amplis', 'ampw.pick': 'Ampli affiché par le widget',
   'ampw.showHint': 'Amplificateur · cliquer pour afficher', 'ampw.hideHint': 'Amplificateur — affiché · cliquer pour masquer',
-  'ampw.close': 'Fermer', 'ampw.offline': 'hors ligne', 'ampw.none': 'Aucun amplificateur configuré',
+  'ampw.close': 'Fermer', 'ampw.offline': 'hors ligne', 'ampw.kpaClear': 'OPERATE efface ce défaut', 'ampw.none': 'Aucun amplificateur configuré',
   'ampw.operate': 'Operate', 'ampw.standby': 'Standby', 'ampw.on': 'ON', 'ampw.off': 'OFF',
   'ampw.lvlL': 'Low', 'ampw.lvlM': 'Mid', 'ampw.lvlH': 'High',
   'ampw.pwr': 'Watts', 'ampw.swr': 'ROS', 'ampw.temp': 'Temp', 'ampw.id': 'Id (A)', 'ampw.fan': 'Mode ventil.',