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/CuePlayer.cs

@@ -20,12 +20,22 @@ namespace RemSound.App;
 /// </summary>
 internal sealed class CuePlayer : IDisposable
 {
+    /// <summary>The "this is a silent / automated launch" flag, set once at startup by the
+    /// <c>--silent</c> flag. Primarily an app-wide mute for every cue sound (startup, connect,
+    /// checkbox, tab-switch, send/receive, previews — everything that plays through a CuePlayer).
+    /// It ALSO doubles as the signal to suppress the unattended startup pop-ups that would otherwise
+    /// ding at and bother whoever's at the screen during a throwaway test launch — the missing-sound
+    /// warning and the Realtek-ASIO-detected warning both check it. So a <c>--silent</c> launch is
+    /// completely quiet: no cue audio, no warning dings, no dialogs. Off in normal use.</summary>
+    public static bool GloballyMuted { get; set; }
+
     private readonly string filePath;
 
     public CuePlayer(string filePath) => this.filePath = filePath;
 
     public void Play()
     {
+        if (GloballyMuted) return;
         var path = filePath;
         Task.Run(() =>
         {