feat(sat): a sky plot — the pass seen from underneath it

The map answers "where is the satellite over the earth". This answers
"where do I look", which during a pass is the question that matters.

The projection is the one every tracker uses and every operator already
reads: the centre is the zenith, the rim is the horizon, north is up. So
the radius is (90 − elevation), not the elevation — a bird overhead is a
dot in the middle, and a pass that hugs the rim is one that never rises.
Whether it comes over the roof or along the treeline is something no
amount of azimuth and elevation digits conveys, and one glance settles.

The whole pass is drawn: a dashed track with arrowheads for the direction
of travel, a hollow circle where it rises, a filled one where it sets,
and a cross where the satellite is now — green above the horizon, grey
below, because the numbers are still right down there and nothing can be
worked through the earth.

The track is fetched once a minute, not once a second: the SHAPE of a
pass does not change while it happens. Only the marker moves, and that
rides on the tuning poll that was already running.

frontend/src/lib/i18n.tsx

@@ -598,6 +598,8 @@ const en: Dict = {
   'sat.hideSide': 'Hide the readout — all map', 'sat.showSide': 'Show the readout',
   'sat.sideWidthTip': 'Drag to resize the readout. Double-click to reset it.',
   'sat.thSat': 'Satellite', 'sat.thAos': 'Rise', 'sat.thLos': 'Set', 'sat.thMaxEl': 'Max', 'sat.thIn': 'In',
+  'sat.skyPlot': 'The pass across the sky — centre is straight up, the rim is the horizon, north is at the top',
+  'sat.showSky': 'Show the sky plot', 'sat.hideSky': 'Hide the sky plot',
   'sec.satellite': 'Satellites',
   'satset.hint': 'Where the antenna is, and the machine that points it. The satellites you follow and the frequency plan are in the Satellites tab.',
   'satset.gridPlaceholder': 'e.g. JN18cx',
@@ -1190,6 +1192,8 @@ const fr: Dict = {
   'sat.hideSide': 'Masquer le panneau — carte plein écran', 'sat.showSide': 'Afficher le panneau',
   'sat.sideWidthTip': 'Glisser pour redimensionner le panneau. Double-clic pour le remettre par défaut.',
   'sat.thSat': 'Satellite', 'sat.thAos': 'Lever', 'sat.thLos': 'Coucher', 'sat.thMaxEl': 'Max', 'sat.thIn': 'Dans',
+  'sat.skyPlot': 'Le passage dans le ciel — le centre est à la verticale, le bord est l’horizon, le nord en haut',
+  'sat.showSky': 'Afficher la vue du ciel', 'sat.hideSky': 'Masquer la vue du ciel',
   'sec.satellite': 'Satellites',
   'satset.hint': 'Où se trouve l’antenne, et la machine qui la pointe. Les satellites suivis et le plan de fréquences sont dans l’onglet Satellites.',
   'satset.gridPlaceholder': 'ex. JN18cx',