Preferences: four accessible tabs (General / Audio cues / Startup behaviour / Update settings)

The Preferences dialog is now a QuietTabControl with four tabs (same accessible tab approach as
the main window; Ctrl+Tab / arrows switch tabs):

- General: profiles-folder browse, accept-remote-volume, UPnP, enable logs / write logs now.
- Audio cues: the redesigned cue UI (plain cue list + "(none)" sound option) + keyboard clicks.
- Startup behaviour: Start minimised / Start with Windows / Start with a specific profile -
  moved here from the standalone Options-menu dialog, wiring and persistence unchanged (AppConfig
  + the Windows auto-start registry entry).
- Update settings: startup-check, frequency, check-now, silent-install, show-what's-new.

Removed the Options-menu "Startup behaviour" item and deleted the now-unused
StartupBehaviourDialog.cs (and dropped it from the self-test's accessibility audit). The audit
still passes on the tabbed dialog with no mnemonic clashes (Alt-letters are isolated per tab).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/RemSound.App/SelfTest.cs

@@ -333,7 +333,6 @@ internal static class SelfTest
     {
         var factories = new (string Name, Func<Form> Make)[]
         {
-            ("Startup behaviour", () => new StartupBehaviourDialog(null)),
             ("Recording settings", () => new RecordingSettingsDialog(new RecordingSettings())),
             ("Preferences", () => new PreferencesDialog(
                 new RemSoundSettingsStore("RemSound"), null,