feat(ftmap): the live decode feed as a world map — feature branch

A new FT Map tab (Tools menu, beside FT decodes): every station decoded in
the last half hour drawn as a great-circle arc from the operator's own
square, coloured by band the way PSK Reporter taught everyone to read it,
fading with age, one arc per call (its freshest sighting), a tooltip with
call/grid/SNR, the MainMap basemap picker and a legend of only the bands on
screen.

Performance was the open question and it is answered structurally: the panel
mounts only while its tab is ACTIVE (zero cost otherwise), the map renders
canvas (one element, not one node per arc), arcs are capped at 300, and
redraws follow the decode list — every period, not every frame.

frontend/src/lib/i18n.tsx

@@ -162,7 +162,7 @@ const en: Dict = {
   'mx.tipThisCall': 'already worked with this callsign',
   'mx.tipThisCallConf': 'already confirmed with this callsign',
   // FTx decodes panel (Tools -> FT decodes)
-  'dec.tab': 'FT decodes', 'dec.title': 'Decodes', 'dec.cqOnly': 'CQ only',
+  'ftmap.tab': 'FT Map', 'ftmap.noGrid': 'Set your station grid in Preferences to place the map.', 'dec.tab': 'FT decodes', 'dec.title': 'Decodes', 'dec.cqOnly': 'CQ only',
   'dec.allBands': 'All bands', 'dec.allModes': 'All modes', 'toast.qsoLogged': 'QSO logged', 'dec.contsHint': 'Continents: click to keep one or several', 'dec.allConts': 'All continents',
   'dec.minSnrTitle': 'Hide anything weaker than this SNR', 'dec.searchPh': 'Call, grid or message',
   'dec.clearFilters': 'Clear', 'dec.live': 'live', 'dec.tx': 'TX',
@@ -682,7 +682,7 @@ const fr: Dict = {
   'mx.tipThisCall': 'déjà contacté avec cet indicatif',
   'mx.tipThisCallConf': 'déjà confirmé avec cet indicatif',
   // Panneau des decodes FTx (Outils -> Decodes FT)
-  'dec.tab': 'Decodes FT', 'dec.title': 'Decodes', 'dec.cqOnly': 'CQ seulement',
+  'ftmap.tab': 'FT Map', 'ftmap.noGrid': 'Renseigne ton locator dans les Préférences pour placer la carte.', 'dec.tab': 'Decodes FT', 'dec.title': 'Decodes', 'dec.cqOnly': 'CQ seulement',
   'dec.allBands': 'Toutes bandes', 'dec.allModes': 'Tous modes', 'toast.qsoLogged': 'QSO enregistré', 'dec.contsHint': 'Continents : clique pour en garder un ou plusieurs', 'dec.allConts': 'Tous continents',
   'dec.minSnrTitle': 'Masquer tout ce qui est plus faible que ce rapport', 'dec.searchPh': 'Indicatif, locator ou message',
   'dec.clearFilters': 'Effacer', 'dec.live': 'en direct', 'dec.tx': 'TX',