Add dark mode toggle under User Preferences > Application

- Add darkMode boolean preference to ApplicationPreference (persisted via java.util.prefs)
- Add ToggleSwitch labeled "Dark Mode" to ApplicationPreferenceEditor
- Add ThemeManager singleton that listens for preference updates and applies/removes
  the dark stylesheet to all registered JavaFX Scenes and swaps the Swing LAF
- Add sdrtrunk_dark.css with dark styling for controls, tables, menus, dialogs, charts
- Add FlatLaf dependency for full Swing dark theming (replaces previous OS-specific
  MetalLookAndFeel setup; FlatLaf works cross-platform)
- Register all Stages created in JavaFxWindowManager with the ThemeManager and walk
  Window.getWindows() at apply-time so unregistered dialogs/alerts also pick up the theme
- Add Foojay toolchain resolver to settings.gradle so Gradle can auto-provision JDK 25

settings.gradle

@@ -1 +1,5 @@
-rootProject.name = 'sdr-trunk'
+plugins {
+    id 'org.gradle.toolchains.foojay-resolver-convention' version '1.0.0'
+}
+
+rootProject.name = 'sdr-trunk'