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

build.gradle

@@ -93,6 +93,7 @@ dependencies {
     implementation 'ch.qos.logback:logback-core:1.5.21'
     implementation 'com.fasterxml.jackson.dataformat:jackson-dataformat-xml:2.20.1'
     implementation 'com.fazecast:jSerialComm:2.11.4'
+    implementation 'com.formdev:flatlaf:3.5.4'
     implementation 'com.github.jiconfont:jiconfont-font_awesome:4.7.0.1'
     implementation 'com.github.jiconfont:jiconfont-javafx:1.0.0'
     implementation 'com.github.jiconfont:jiconfont-swing:1.0.1'