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>

build-release.ps1

@@ -37,10 +37,10 @@ $staging = Join-Path ([System.IO.Path]::GetTempPath()) ("remsound-release-" + [g
 
 # 0a. SoundForge drops a .sfk peak file next to every .wav it opens. They're byproducts that must
 #     never ship (the build only bundles *.wav, so they wouldn't anyway) - clear them from the
-#     source sounds\ folder so they don't accumulate and clutter the working tree.
-$sfk = @(Get-ChildItem -LiteralPath (Join-Path $repo 'sounds') -Filter '*.sfk' -ErrorAction SilentlyContinue)
+#     source 'default sounds\' folder so they don't accumulate and clutter the working tree.
+$sfk = @(Get-ChildItem -LiteralPath (Join-Path $repo 'default sounds') -Filter '*.sfk' -ErrorAction SilentlyContinue)
 if ($sfk.Count -gt 0) {
-    Write-Host "Removing $($sfk.Count) SoundForge .sfk byproduct(s) from sounds\..." -ForegroundColor Cyan
+    Write-Host "Removing $($sfk.Count) SoundForge .sfk byproduct(s) from 'default sounds\'..." -ForegroundColor Cyan
     $sfk | Remove-Item -Force
 }