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_tab.py

@@ -13,7 +13,7 @@ from PyQt5.QtWidgets import (
     QSplitter, QGroupBox, QFrame, QCheckBox, QSpinBox,
 )
 from PyQt5.QtCore import Qt, QTimer
-from PyQt5.QtGui  import QColor, QBrush
+from PyQt5.QtGui  import QColor, QBrush, QIcon, QPixmap
 
 from data_model import ReceiverManager, GNSS_NAMES, GNSS_COLORS
 from sky_widget import SkyPlotWidget
@@ -37,6 +37,8 @@ class SkyTab(QWidget):
         self._name_labels: dict = {}   # rid -> QLabel (receiver display name)
         self._trail:       dict = {}   # (rid, gnss_id, sv_id) -> deque[(elev, azim)]
         self._hi_timers:   dict = {}   # (rid, gnss_id, sv_id) -> QTimer
+        self._gnss_checks: dict = {}   # gnss_id -> QCheckBox
+        self._enabled_gnss: set = set(GNSS_NAMES.keys())
 
         outer = QVBoxLayout(self)
         outer.setContentsMargins(4, 4, 4, 4)
@@ -68,7 +70,28 @@ class SkyTab(QWidget):
         self._trail_spin.setEnabled(False)
         self._trail_spin.valueChanged.connect(self._on_trail_toggled)
         src_bl.addWidget(self._trail_spin)
-        outer.addWidget(src_box)
+
+        # ── Constellation legend / filter ─────────────────────────────────────
+        const_box = QGroupBox("Constellations  (check to show)")
+        const_box.setMaximumHeight(80)
+        const_bl  = QHBoxLayout(const_box)
+        const_bl.setContentsMargins(4, 2, 4, 2)
+        const_bl.setSpacing(8)
+        for gid in sorted(GNSS_NAMES):
+            cb = QCheckBox(GNSS_NAMES[gid])
+            cb.setChecked(True)
+            pix = QPixmap(10, 10)
+            pix.fill(QColor(GNSS_COLORS.get(gid, '#9E9E9E')))
+            cb.setIcon(QIcon(pix))
+            cb.toggled.connect(lambda checked, g=gid: self._on_gnss_toggled(g, checked))
+            self._gnss_checks[gid] = cb
+            const_bl.addWidget(cb)
+        const_bl.addStretch()
+
+        filter_row = QHBoxLayout()
+        filter_row.addWidget(src_box)
+        filter_row.addWidget(const_box)
+        outer.addLayout(filter_row)
 
         # ── Info bar (one card per receiver) ──────────────────────────────────
         info_frame = QFrame()
@@ -181,7 +204,7 @@ class SkyTab(QWidget):
     def _visible_sats(self) -> list:
         selected = self._selected_rids()
         return [(k[0], self._store[k])
-                for k in self._store if k[0] in selected]
+                for k in self._store if k[0] in selected and k[1] in self._enabled_gnss]
 
     def _active_trails(self) -> dict:
         if not self._trail_cb.isChecked():
@@ -192,7 +215,14 @@ class SkyTab(QWidget):
         selected = self._selected_rids()
         return {key: list(dq)[-n_points:]
                 for key, dq in self._trail.items()
-                if key[0] in selected}
+                if key[0] in selected and key[1] in self._enabled_gnss}
+
+    def _on_gnss_toggled(self, gid: int, checked: bool):
+        if checked:
+            self._enabled_gnss.add(gid)
+        else:
+            self._enabled_gnss.discard(gid)
+        self._refresh_immediate()
 
     # ── Trail toggle ──────────────────────────────────────────────────────────
 

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 ────────────────────────────────────────────────────────────────────