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/SelfTest.cs

@@ -295,12 +295,11 @@ internal static class SelfTest
         var root = AppContext.BaseDirectory;
         Check(File.Exists(Path.Combine(root, "readme.html")), "readme.html (the F1 manual) must ship next to the exe");
 
-        // Cues are consolidated from the shipped sounds\ folder into the runtime sounds folder at
-        // startup (Program.ConsolidateSounds), so by the time the self-test runs they live here.
-        // An empty runtime folder means the shipped build had no sounds to seed from - exactly the
-        // bug that shipped the v3.9 zip with no cue sounds.
+        // The shipped DEFAULT cues live install-side in "default sounds\" next to the exe
+        // (AppConfig.SoundsDirectory). An empty/absent folder means the shipped build had no sounds -
+        // exactly the bug that shipped the v3.9 zip with no cue sounds.
         var soundsDir = AppConfig.SoundsDirectory;
-        Check(Directory.Exists(soundsDir), "the runtime sounds folder must exist (cues are consolidated at startup)");
+        Check(Directory.Exists(soundsDir), "the shipped 'default sounds' folder must exist next to the exe");
         // Cues ship as numbered variants ("connect 1.wav", ...); each required cue must have at
         // least one variant present.
         foreach (var cue in new[]
@@ -311,7 +310,7 @@ internal static class SelfTest
         })
         {
             Check(CueSounds.Variants(cue).Count > 0,
-                $"no sound variant present for the '{Path.GetFileNameWithoutExtension(cue)}' cue (was the shipped sounds\\ folder empty?)");
+                $"no sound variant present for the '{Path.GetFileNameWithoutExtension(cue)}' cue (was the shipped 'default sounds' folder empty?)");
         }
         // Keyboard-click typing sounds + the password passkey sound.
         Check(File.Exists(Path.Combine(soundsDir, "key 1.wav")), "keyboard-click sound 'key 1.wav' must be present");