gui: add checkable constellation legend to filter sky plot by GNSS system

Replaces the static painted legend with real checkboxes (one per
GNSS system, color-swatched to match GNSS_COLORS) so users can
toggle which constellations show in the sky plot and satellite
table, mirroring the existing receiver-source filter checkboxes.

gui/sky_widget.py

@@ -11,7 +11,7 @@ from PyQt5.QtWidgets import QWidget, QSizePolicy, QApplication
 from PyQt5.QtGui     import (QPainter, QColor, QPen, QBrush, QFont, QImage)
 from PyQt5.QtCore    import Qt, QPointF, QRectF, QThread, pyqtSignal
 
-from data_model import GNSS_NAMES, GNSS_COLORS, GNSS_SHORT
+from data_model import GNSS_COLORS, GNSS_SHORT
 
 # ── Static colors (created once in main thread, read-only in render thread) ──
 _C_BG      = QColor('#1a1a2e')
@@ -92,7 +92,6 @@ class SkyRenderThread(QThread):
         if trails:
             self._draw_trails(p, cx, cy, max_r, trails)
         self._draw_satellites(p, cx, cy, max_r, sats)
-        self._draw_legend(p, sats)
 
         p.end()
         return img
@@ -184,24 +183,6 @@ class SkyRenderThread(QThread):
             p.drawText(QRectF(x + size + 1, y - 6, 28, 12),
                        Qt.AlignLeft, f"{short}{sat.sv_id}")
 
-    def _draw_legend(self, p: QPainter, sats):
-        used = {sat.gnss_id for _, sat in sats}
-        if not used:
-            return
-        f = QFont(); f.setPointSize(8)
-        p.setFont(f)
-        lx, ly = 8, 8
-        for gid in sorted(used):
-            color = _GNSS_QCOLORS.get(gid)
-            if color is None:
-                continue
-            p.setBrush(QBrush(color))
-            p.setPen(Qt.NoPen)
-            p.drawEllipse(QPointF(lx + 5, ly + 6), 5, 5)
-            p.setPen(QPen(_C_SAT_LBL))
-            p.drawText(lx + 14, ly + 11, GNSS_NAMES.get(gid, f'SYS{gid}'))
-            ly += 18
-
 
 # ── Widget ────────────────────────────────────────────────────────────────────