fix(amp): couple a chosen GROUP, not every amplifier

The first version was a single switch meaning "command them all", and that is
wrong the moment a station has three: two SPE on a combiner and a PowerGenius
on another antenna would all go into OPERATE together, keying an amplifier that
has nothing to do with the pair.

It is now a set. Each amplifier is ticked into the group or not, the group is
stored as a list of ids, and an amplifier outside it keeps its own buttons —
which is the entire point of it being a set.

A group of fewer than two members is stored as none: one amplifier coupled to
itself would make every command fan out to a single member for ever.

A remembered member that is no longer running is skipped rather than failing
the command — deleting one amplifier must not break the button on the other.

An amplifier saved without an id cannot join, and the panel says so instead of
quietly omitting it from the list.

changelog.json

@@ -3,10 +3,10 @@
     "version": "0.25.1",
     "date": "",
     "en": [
-      "Amplifiers: an option commands them together — ON, OFF and OPERATE act on all at once, for a combiner. Each keeps its own meters."
+      "Amplifiers: tick the ones sharing a combiner and ON, OFF and OPERATE act on all of them at once. Each keeps its own meters."
     ],
     "fr": [
-      "Amplificateurs : une option les commande ensemble — ON, OFF et OPERATE agissent sur tous à la fois, pour un combiner. Chacun garde ses mesures."
+      "Amplificateurs : coche ceux qui partagent un combiner et ON, OFF et OPERATE agissent sur tous à la fois. Chacun garde ses mesures."
     ]
   },
   {