Add preset themes selector under User Preferences > Application

Replace the single Dark Mode toggle with a curated selection of 9 preset
themes, exposed as a vertical radio-button group:

  Light                IntelliJ              Nord
  Dark                 Darcula               Dracula
  Solarized Light      Solarized Dark        Monokai Pro

Each Theme enum value pairs a human-readable display name with a FlatLaf
class name and a dark flag.  The first four come from the core flatlaf
dependency; the others come from flatlaf-intellij-themes which is added
as a new implementation dependency (~1MB).

ApplicationPreference now stores the selected theme by enum name under
the new ui.theme key.  isDarkMode() is retained as a compatibility shim
that delegates to the current theme.  Existing users with the legacy
dark.mode boolean are auto-migrated on first read.

ThemeManager is refactored to drive the LAF from the Theme:
- Reflectively instantiates the FlatLaf class.
- Reads Panel.background, Button.background, TextField.background,
  List.selectionBackground/Foreground, Label.foreground/disabledForeground,
  Component.borderColor and Table.alternateRowColor from UIManager *after*
  FlatLaf installs, then seeds those values into the JIDE-specific keys.
  Each theme automatically contributes its own palette to JIDE widgets
  without per-theme hardcoded colours.

ApplicationPreferenceEditor swaps the ToggleSwitch for a ToggleGroup of
RadioButtons that persist the selection through ApplicationPreference.setTheme.

build.gradle

@@ -94,6 +94,7 @@ dependencies {
     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.formdev:flatlaf-intellij-themes: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'