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/FormLayoutRows.cs

@@ -74,6 +74,17 @@ internal sealed class QuietTabControl : TabControl
     protected override AccessibleObject CreateAccessibilityInstance()
         => new QuietAcc(this);
 
+    protected override void OnSelectedIndexChanged(EventArgs e)
+    {
+        base.OnSelectedIndexChanged(e);
+        // Audible tab-switch cue, app-wide (every QuietTabControl gets it for free). Gated on
+        // ContainsFocus so a genuine user switch - arrow keys with the strip focused, or Ctrl+Tab
+        // with focus on a control inside the active page - clicks, while the programmatic
+        // SelectedIndex set done while a window is being built or its last tab restored (nothing
+        // here focused yet) stays silent.
+        if (ContainsFocus) TabSwitchSoundService.Play();
+    }
+
     private sealed class QuietAcc : ControlAccessibleObject
     {
         public QuietAcc(Control owner) : base(owner) { }