fix(worked-before): a prefixed call is one operator, not a whole country

Reported from a screenshot: working ZA/OE8NDR, the worked-before list showed
ZA/IZ2DPX and ZA/IW2JOP beside him and the header counted all four as contacts
"with this call".

callMatch folded portable forms together by taking the part before the slash.
That is right when the slash carries a suffix — RK3DWA/3, RK3DWA/P are one
person — and exactly wrong when it carries a prefix: ZA/OE8NDR became the
station "ZA", and the predicate `callsign LIKE 'ZA/%'` then selected every other
visitor to Albania.

The operator's own call is now picked out properly: drop the known qualifiers
(P, M, MM, AM, QRP, a bare call-area digit) and of what remains take the longest
part — a country prefix is short by nature, ZA, F, KH6, VP2E, and a callsign is
not. The predicate matches the prefixed forms too, so ZA/OE8NDR and a plain
OE8NDR still find each other, which is the whole point of the fold.

A base under three characters falls back to an exact match: a malformed entry
must not produce a LIKE that selects half the logbook.

changelog.json

@@ -4,11 +4,13 @@
     "date": "",
     "en": [
       "Opening the Awards panel no longer pulls the whole logbook several times at once — a large log briefly took gigabytes of memory.",
-      "Awards: the Missing refs button now only appears where it means something — a worldwide award like POTA could only ever answer “nothing found”."
+      "Awards: the Missing refs button now only appears where it means something — a worldwide award like POTA could only ever answer “nothing found”.",
+      "Worked before: a prefixed call like ZA/OE8NDR matched every other visitor to that country instead of that one operator."
     ],
     "fr": [
       "Ouvrir le panneau Awards ne tire plus plusieurs fois le journal entier en même temps — un gros log occupait brièvement des gigaoctets de mémoire.",
-      "Awards : le bouton Réf. manquantes n’apparaît plus que là où il a un sens — un diplôme mondial comme POTA ne pouvait répondre que « aucun manque »."
+      "Awards : le bouton Réf. manquantes n’apparaît plus que là où il a un sens — un diplôme mondial comme POTA ne pouvait répondre que « aucun manque ».",
+      "Déjà contacté : un indicatif préfixé comme ZA/OE8NDR rapprochait tous les autres visiteurs du pays au lieu de ce seul opérateur."
     ]
   },
   {