feat(bandopen): announce sporadic-E openings on 6, 4 and 2 m

Observation, not prediction, and it needs no new data source: the cluster event
worker already enriches every spot with the great-circle distance and bearing
from the operator's grid, which is exactly what a single-hop Es detection rests
on.

The signature is four or more DISTINCT stations at 500-2400 km inside a 90
degree bearing sector within twelve minutes. Each constraint earns its place:
distinct callsigns because one station spotted by six skimmers is six spots and
one station; the lower bound because a 6 m contact under 500 km is ordinary
tropo and says nothing about the ionosphere; the upper bound because past one
hop the bearing test stops meaning anything; and the sector because a real Es
cloud illuminates a direction, which is what separates an opening from a merely
busy evening.

Fed AFTER the Historical guard in the worker. A SH/DX reply replays a hundred
past spots in a second - precisely the shape of a burst - and would announce an
opening that ended hours ago.

Season LABELS, it never gates. Both hemispheres get a summer peak and a lesser
winter one, and an opening outside those is announced with "unusual for the
season" attached: the rare one is the one an operator must not hear about last.

One announcement per band per opening (45 minute quiet period). An opening runs
for hours and produces hundreds of spots; one alert is information, forty is
noise.

frontend/src/App.tsx

@@ -2077,6 +2077,23 @@ export default function App() {
     return () => { off(); };
   }, [showToast]);
 
+  // Band openings — sporadic E on 6/4/2 m, detected from the spot stream.
+  //
+  // A toast rather than a rule-based alert: this is not "a station you wanted
+  // appeared", it is "the band itself just changed", and it fires a handful of
+  // times a season. The detector already announces each band once per opening,
+  // so there is nothing to throttle here.
+  useEffect(() => {
+    const off = EventsOn('bandopen:detected', (o: any) => {
+      if (!o?.band) return;
+      const season = o.in_season ? '' : ` — ${t('bmp.openUnusual')}`;
+      showToast(`📡 ${t('bmp.openToast', {
+        band: String(o.band).toUpperCase(), n: o.calls, km: o.median_km,
+      })}${season}`);
+    });
+    return () => { off(); };
+  }, [showToast, t]);
+
   // DX-cluster spot alerts: a matched rule fires here. Play a beep (WebAudio, no
   // asset needed — CSP-safe) and/or show a toast, per the rule's chosen actions.
   useEffect(() => {

frontend/src/lib/i18n.tsx

@@ -343,7 +343,7 @@ const en: Dict = {
   'bmp.statusWorked': 'Worked (this band + mode already in log)', 'bmp.statusUnresolved': 'Entity not resolved', 'bmp.bandMap': 'Band map', 'bmp.notConfigured': 'Not configured for {band}.',
   'bmp.map': 'Map', 'bmp.zoomOut': 'Zoom out', 'bmp.zoomIn': 'Zoom in', 'bmp.scrollToRig': 'Scroll to current rig frequency', 'bmp.moveLeft': 'Move band map to the left', 'bmp.moveRight': 'Move band map to the right', 'bmp.hide': 'Hide band map',
   'bmp.bandsLabel': 'Bands:', 'bmp.fit': 'FIT', 'bmp.hideFt': 'Hide FTx', 'bmp.hideFtTitle': 'Hide all digital (FT8/FT4/JS8/…) spots on every band map', 'bmp.fitBand': 'Fit to band', 'bmp.widthTip': 'Drag to resize — double-click to reset', 'bmp.fitTitle': 'Size each band map to show the whole band edge-to-edge',
-  'bmp.legendNewDxcc': 'New DXCC', 'bmp.legendNewBand': 'New band', 'bmp.legendNewSlot': 'New slot (mode)', 'bmp.legendWorked': 'Worked', 'bmp.legendNewPota': 'New POTA', 'bmp.legendNewCounty': 'New county', 'bmp.legendWorkedCall': 'Callsign already worked', 'bmp.legendCW': 'CW', 'bmp.legendData': 'Data', 'bmp.legendPhone': 'Phone', 'bmp.footerHint': 'scroll · ctrl+wheel = zoom · ◎ = jump to rig', 'bmp.spotsHidden': '{n} FT8/FT4 spots hidden — top {max} kept (CW/SSB all shown)',
+  'bmp.legendNewDxcc': 'New DXCC', 'bmp.legendNewBand': 'New band', 'bmp.legendNewSlot': 'New slot (mode)', 'bmp.openToast': '{band} is open — {n} stations around {km} km', 'bmp.openUnusual': 'unusual for the season', 'bmp.legendWorked': 'Worked', 'bmp.legendNewPota': 'New POTA', 'bmp.legendNewCounty': 'New county', 'bmp.legendWorkedCall': 'Callsign already worked', 'bmp.legendCW': 'CW', 'bmp.legendData': 'Data', 'bmp.legendPhone': 'Phone', 'bmp.footerHint': 'scroll · ctrl+wheel = zoom · ◎ = jump to rig', 'bmp.spotsHidden': '{n} FT8/FT4 spots hidden — top {max} kept (CW/SSB all shown)',
   'frm.welcome': 'Welcome to OpsLog', 'frm.intro': 'Set up your station to start logging. These fields stamp every QSO and can be changed later in Preferences → Station Information (and per profile).',
   'frm.callsign': 'Callsign', 'frm.locator': 'Locator', 'frm.operator': 'Operator', 'frm.operatorPh': 'same as callsign', 'frm.owner': 'Owner', 'frm.ownerPh': 'station owner callsign', 'frm.name': 'Name', 'frm.namePh': 'your first name',
   'frm.awardRefs': 'Award reference lists', 'frm.awardRefsHint': 'IOTA · POTA · WWFF · SOTA — names & totals for those awards (optional, can take a minute).', 'frm.downloading': 'Downloading…', 'frm.reDownload': 'Re-download', 'frm.download': 'Download', 'frm.required': 'Callsign and locator are required.', 'frm.saving': 'Saving…', 'frm.startLogging': 'Start logging',
@@ -751,7 +751,7 @@ const fr: Dict = {
   'bmp.statusWorked': 'Contacté (cette bande + mode déjà au log)', 'bmp.statusUnresolved': 'Entité non résolue', 'bmp.bandMap': 'Carte de bande', 'bmp.notConfigured': 'Non configurée pour {band}.',
   'bmp.map': 'Carte', 'bmp.zoomOut': 'Dézoomer', 'bmp.zoomIn': 'Zoomer', 'bmp.scrollToRig': 'Aller à la fréquence actuelle du poste', 'bmp.moveLeft': 'Déplacer la carte de bande à gauche', 'bmp.moveRight': 'Déplacer la carte de bande à droite', 'bmp.hide': 'Masquer la carte de bande',
   'bmp.bandsLabel': 'Bandes :', 'bmp.fit': 'FIT', 'bmp.hideFt': 'Masquer FTx', 'bmp.hideFtTitle': 'Masquer tous les spots numériques (FT8/FT4/JS8/…) sur toutes les cartes', 'bmp.fitBand': 'Ajuster à la bande', 'bmp.widthTip': 'Glisser pour redimensionner — double-clic pour réinitialiser', 'bmp.fitTitle': 'Dimensionner chaque carte pour afficher toute la bande',
-  'bmp.legendNewDxcc': 'Nouveau DXCC', 'bmp.legendNewBand': 'Nouvelle bande', 'bmp.legendNewSlot': 'Nouveau mode', 'bmp.legendWorked': 'Contacté', 'bmp.legendNewPota': 'Nouveau POTA', 'bmp.legendNewCounty': 'Nouveau comté', 'bmp.legendWorkedCall': 'Indicatif déjà contacté', 'bmp.legendCW': 'CW', 'bmp.legendData': 'Numérique', 'bmp.legendPhone': 'Phonie', 'bmp.footerHint': 'défiler · ctrl+molette = zoom · ◎ = aller au poste', 'bmp.spotsHidden': '{n} spots FT8/FT4 masqués — {max} meilleurs conservés (CW/SSB tous affichés)',
+  'bmp.legendNewDxcc': 'Nouveau DXCC', 'bmp.legendNewBand': 'Nouvelle bande', 'bmp.legendNewSlot': 'Nouveau mode', 'bmp.openToast': 'Ouverture {band} — {n} stations vers {km} km', 'bmp.openUnusual': 'inhabituel pour la saison', 'bmp.legendWorked': 'Contacté', 'bmp.legendNewPota': 'Nouveau POTA', 'bmp.legendNewCounty': 'Nouveau comté', 'bmp.legendWorkedCall': 'Indicatif déjà contacté', 'bmp.legendCW': 'CW', 'bmp.legendData': 'Numérique', 'bmp.legendPhone': 'Phonie', 'bmp.footerHint': 'défiler · ctrl+molette = zoom · ◎ = aller au poste', 'bmp.spotsHidden': '{n} spots FT8/FT4 masqués — {max} meilleurs conservés (CW/SSB tous affichés)',
   'frm.welcome': 'Bienvenue dans OpsLog', 'frm.intro': 'Configure ta station pour commencer à logger. Ces champs sont inscrits sur chaque QSO et peuvent être modifiés plus tard dans Préférences → Informations station (et par profil).',
   'frm.callsign': 'Indicatif', 'frm.locator': 'Locator', 'frm.operator': 'Opérateur', 'frm.operatorPh': "identique à l'indicatif", 'frm.owner': 'Propriétaire', 'frm.ownerPh': 'indicatif du propriétaire de la station', 'frm.name': 'Nom', 'frm.namePh': 'ton prénom',
   'frm.awardRefs': 'Listes de références des diplômes', 'frm.awardRefsHint': 'IOTA · POTA · WWFF · SOTA — noms et totaux pour ces diplômes (optionnel, peut prendre une minute).', 'frm.downloading': 'Téléchargement…', 'frm.reDownload': 'Retélécharger', 'frm.download': 'Télécharger', 'frm.required': "L'indicatif et le locator sont obligatoires.", 'frm.saving': 'Enregistrement…', 'frm.startLogging': 'Commencer à logger',