"Use Windows default" is now exclusive: it clears + locks out the specific cards

Ed: ticking "Use Windows default audio device" should uncheck the other cards and stop
them being ticked again until the default is turned back off - in both the app and the
service. This replaces the old optional "shall I untick the others?" prompt with a hard
rule.

Behaviour (all follower lists: app received-outputs, outputs-to-send and inputs-to-send;
service outputs):
- Ticking the follower unticks every specific card in that list.
- While the follower is on, trying to tick a specific card is refused (the ItemCheck is
  vetoed straight back to unticked) - you're either following the Windows default or
  picking cards, never both.
- Unticking the follower frees the cards again.
- Enforced on load too, so a hand-edited/legacy profile carrying both comes up clean.

Shared, not duplicated: the veto + clear + follower-checked helpers live in
AudioDefaultFollower and are used by MainForm and the service dialog alike.

Removed the now-obsolete soft prompt and everything that hung off it:
- MainForm.MaybeUntickOthersForDefault + ResetDefaultAudioDevicePrompt.
- The "Reset the default audio device prompt" Options item (and its Alt+O,R).
- AppConfig.UntickOthersWhenUsingDefaultOutput/Input.
- Manual + About-box references to the prompt / reset item; the manual now documents
  the exclusive rule and lists all three follower lists (the outputs-to-send follower
  had been undocumented).

Test: "Default follower exclusivity" self-test - follower on clears + locks specific
cards, follower off frees them, unticking and the follower entry itself are never
vetoed. Gate 45/45.

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

src/RemSound.Core/AppConfig.cs

@@ -248,12 +248,6 @@ public sealed class AppConfig
     /// device, following it if the default changes. Machine-wide, opt-in, default false.</summary>
     public bool UseDefaultLoopbackSend { get; set; }
 
-    /// <summary>Remembered answer to the "untick the other outputs/inputs when you turn on Use Windows
-    /// default?" prompt. Null = ask each time; true = always untick; false = never untick. Set when the
-    /// user ticks "Don't ask again" on that prompt. Machine-wide.</summary>
-    public bool? UntickOthersWhenUsingDefaultOutput { get; set; }
-    public bool? UntickOthersWhenUsingDefaultInput { get; set; }
-
     /// <summary>If non-null and a profile with this title exists, RemSound skips the
     /// startup profile picker and loads this profile directly. Combine with
     /// <see cref="StartMinimised"/> + the Windows auto-start registry entry