Persist settings per-computer; add Reset to Defaults in General tab

- New core/app_settings.py: load/save settings.json to platform config dir
  (Windows: %APPDATA%, Linux: ~/.config, macOS: ~/Library/Application Support)
- MainWindow loads saved settings on startup, saves on every Apply & Close
- Settings survive profile switches and app restarts independently of profiles
- Settings > General: Reset to Defaults button with QMessageBox confirmation

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

2 files changed, 30 insertions, 2 deletions

diff --git a/ui/main_window.py b/ui/main_window.py
index 7527a8a..ea299e7 100644
--- a/ui/main_window.py
+++ b/ui/main_window.py
@@ -45,6 +45,7 @@ from ui.windows.settings_window    import SettingsWindow
 
 from plugins.plugin_manager import PluginManager
 from plugins.base_plugin    import PluginContext
+from core.app_settings      import load_settings, save_settings
 
 
 _DARK_QSS  = os.path.join(os.path.dirname(os.path.abspath(__file__)), "style_dark.qss")
@@ -60,7 +61,7 @@ class MainWindow(QMainWindow):
         self.registry  = DeviceRegistry()
         self.engine    = AcquisitionEngine(poll_interval_ms=100)
         self.processor = SignalProcessor()
-        self._settings = dict(SettingsWindow._defaults)
+        self._settings = load_settings(SettingsWindow._defaults)
 
         self._elapsed          = 0
         self._win_devices      = None
@@ -548,6 +549,7 @@ class MainWindow(QMainWindow):
     def _on_settings(self, cfg: dict):
         self._settings.update(cfg)
         self.engine._interval = cfg.get("poll_ms", 100) / 1000.0
+        save_settings(self._settings)
 
     def _tick(self):
         self._elapsed += 1
diff --git a/ui/windows/settings_window.py b/ui/windows/settings_window.py
index 422b6c9..1630efa 100644
--- a/ui/windows/settings_window.py
+++ b/ui/windows/settings_window.py
@@ -13,7 +13,7 @@ from PyQt6.QtWidgets import (
     QWidget, QVBoxLayout, QHBoxLayout, QLabel, QPushButton,
     QTabWidget, QFrame, QFormLayout, QComboBox,
     QSpinBox, QDoubleSpinBox, QCheckBox, QLineEdit,
-    QFileDialog, QScrollArea, QGroupBox,
+    QFileDialog, QScrollArea, QGroupBox, QMessageBox,
 )
 from PyQt6.QtCore import Qt, pyqtSignal
 from PyQt6.QtGui import QCloseEvent, QFont
@@ -108,10 +108,25 @@ class SettingsWindow(QWidget):
         self._aa_chk = QCheckBox(); self._aa_chk.setChecked(self.cfg["antialias"])
         lay.addRow("Anti-alias plots:", self._aa_chk)
 
+        rst = QPushButton("Reset to Defaults"); rst.setObjectName("configButton")
+        rst.clicked.connect(self._reset_to_defaults)
+        lay.addRow("", rst)
+
         scroll.setWidget(cont)
         root = QVBoxLayout(w); root.setContentsMargins(0,0,0,0); root.addWidget(scroll)
         return w
 
+    def _populate_from_cfg(self):
+        self._theme_cb.setCurrentText(self.cfg["theme"].title())
+        self._font_sp.setValue(self.cfg["font_size"])
+        self._aa_chk.setChecked(self.cfg["antialias"])
+        self._poll_sp.setValue(self.cfg["poll_ms"])
+        self._buf_sp.setValue(self.cfg["buffer_size"])
+        self._log_edit.setText(self.cfg["log_dir"])
+        self._tw_sp.setValue(self.cfg["time_window_s"])
+        self._legend_chk.setChecked(self.cfg["show_legend"])
+        self._grid_chk.setChecked(self.cfg["show_grid"])
+
     def _acquisition_tab(self):
         w = QWidget()
         scroll = QScrollArea(); scroll.setWidgetResizable(True)
@@ -251,6 +266,17 @@ class SettingsWindow(QWidget):
 
     # ── Actions ───────────────────────────────────────────────────────────
 
+    def _reset_to_defaults(self):
+        reply = QMessageBox.question(
+            self, "Reset Settings",
+            "Reset all settings to defaults?\nThis cannot be undone.",
+            QMessageBox.StandardButton.Yes | QMessageBox.StandardButton.No,
+            QMessageBox.StandardButton.No,
+        )
+        if reply == QMessageBox.StandardButton.Yes:
+            self.cfg = dict(self._defaults)
+            self._populate_from_cfg()
+
     def _browse_log(self):
         path = QFileDialog.getExistingDirectory(self, "Select Log Directory")
         if path: