v4.0: full audio-cue system, cause-aware auto-tune, four-tab Preferences, install-side default sounds

Audio cues
- Cues for send/receive on-off, minimise/restore, checkbox tick/untick, and tab switch
- Soft keyboard clicks while typing, with a distinct passkey sound on password fields
- Per-cue "Choose sound" variant picker; "(none)" silences a cue; front-most missing-sound warning
- Send/receive cues take priority over the generic checkbox sound; programmatic ticks stay silent

Preferences
- Redesigned into four tabs (General, Audio cues, Startup behaviour, Update settings)
- Startup behaviour moved in from the Options menu
- NVDA now announces the dialog on open (focus a real named control, not the quiet tab control)

Auto-tune
- Cause-aware: tells device render-callback stalls (more buffer can't fix) apart from genuine
  network/buffer starvation, so it no longer pins latency high on chunky onboard cards
- Lowering the target eases the buffer down (glide) instead of trimming it, so no clicks while tuning

Sounds layout
- Shipped defaults moved out of the per-user folder into an install-side "default sounds" folder,
  so updates can refresh them; user customs are Browse-picked file paths and are left untouched
- Startup migration removes both legacy sound folders; verified from oldest (v1.0-v3.3) and v3.4 layouts

Quiet automated launches
- New --silent launch flag mutes all cue sounds and suppresses the startup dialogs (migration notice,
  update check, Realtek/mic/missing-sound warnings) so test launches never disturb the user
- run-tests / build-release / SelfTest repointed to the new "default sounds" layout

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

src/RemSound.App/CheckSoundService.cs

@@ -17,6 +17,13 @@ internal static class CheckSoundService
     private static CuePlayer? checkSound;
     private static CuePlayer? uncheckSound;
 
+    /// <summary>When true, <see cref="Play"/> is a no-op. MainForm sets this around bulk programmatic
+    /// control updates (profile load, "uncheck all", device-list refresh). The per-call Focused gate
+    /// already silences MOST programmatic ticks, but it leaks when the box we tick in code happens to
+    /// be the focused control on launch — which is exactly what made loading a profile blast a
+    /// checkbox click. This flag closes that gap: only genuine user toggles ever click.</summary>
+    public static bool Suppressed { get; set; }
+
     /// <summary>(Re)load the tick/untick sounds from the current cue configuration. Call at startup
     /// and whenever cue settings change.</summary>
     public static void Reload()
@@ -28,6 +35,7 @@ internal static class CheckSoundService
 
     public static void Play(bool isChecked)
     {
+        if (Suppressed) return;
         var cfg = AppConfig.Load();
         if (isChecked) { if (cfg.EnableCheckboxOnCue) checkSound?.Play(); }
         else { if (cfg.EnableCheckboxOffCue) uncheckSound?.Play(); }