sky tab: extend trail storage to 24h, add visible trail depth spinbox

Trail history increased from 1h to 24h (86400 points). Added a spinbox
(0-24h, default 1h) next to 'Show Trails' to control how much trail is
rendered; spinbox is disabled when trails are off.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui/sky_tab.py

@@ -10,7 +10,7 @@ for _p in [_root, _gui]:
 from PyQt5.QtWidgets import (
     QWidget, QHBoxLayout, QVBoxLayout, QLabel, QListWidget,
     QListWidgetItem, QTableWidget, QTableWidgetItem, QHeaderView,
-    QSplitter, QGroupBox, QFrame, QCheckBox,
+    QSplitter, QGroupBox, QFrame, QCheckBox, QSpinBox,
 )
 from PyQt5.QtCore import Qt, QTimer
 from PyQt5.QtGui  import QColor, QBrush
@@ -19,7 +19,8 @@ from data_model import ReceiverManager, GNSS_NAMES, GNSS_COLORS
 from sky_widget import SkyPlotWidget
 
 _COL_HEADERS  = ['GNSS', 'SV', 'Elev °', 'Az °', 'C/N₀', 'Fix', 'Receiver']
-_TRAIL_MAXLEN = 3600   # 1 hour at 1 Hz
+_TRAIL_MAXLEN    = 86400   # 24 hours at 1 Hz
+_TRAIL_MAX_HOURS = 24
 
 
 class SkyTab(QWidget):
@@ -54,6 +55,15 @@ class SkyTab(QWidget):
         self._trail_cb.setChecked(False)
         self._trail_cb.toggled.connect(self._on_trail_toggled)
         src_bl.addWidget(self._trail_cb)
+
+        self._trail_spin = QSpinBox()
+        self._trail_spin.setRange(0, _TRAIL_MAX_HOURS)
+        self._trail_spin.setValue(1)
+        self._trail_spin.setSuffix(" h")
+        self._trail_spin.setFixedWidth(58)
+        self._trail_spin.setEnabled(False)
+        self._trail_spin.valueChanged.connect(self._on_trail_toggled)
+        src_bl.addWidget(self._trail_spin)
         outer.addWidget(src_box)
 
         # ── Info bar (one card per receiver) ──────────────────────────────────
@@ -154,14 +164,18 @@ class SkyTab(QWidget):
     def _active_trails(self) -> dict:
         if not self._trail_cb.isChecked():
             return {}
+        n_points = self._trail_spin.value() * 3600
+        if n_points == 0:
+            return {}
         selected = self._selected_rids()
-        return {key: list(dq)
+        return {key: list(dq)[-n_points:]
                 for key, dq in self._trail.items()
                 if key[0] in selected}
 
     # ── Trail toggle ──────────────────────────────────────────────────────────
 
-    def _on_trail_toggled(self, _checked: bool):
+    def _on_trail_toggled(self, _value=None):
+        self._trail_spin.setEnabled(self._trail_cb.isChecked())
         self._sky.update_data(self._visible_sats(), self._active_trails())
 
     # ── Info bar ──────────────────────────────────────────────────────────────