Audio cues: per-cue default-sound picker, keyboard-click typing feedback, passkey on password fields

Cue sounds now ship as numbered variants ("connect 1.wav", "connect 2.wav", ...); the
count is never hard-coded so more can be added with no code change.

- CueSounds.cs: discovers a cue's "<base> <n>.wav" variants (case-insensitive) and
  resolves the active default: per-profile custom WAV > machine-wide chosen variant >
  first variant > silent. Wired into MainForm.TryLoadCueSound, the startup cue in
  Program.cs, and PreferencesDialog.ResolveCueFilePath.
- AppConfig: DefaultCueSounds (machine-wide cueId -> chosen filename) and
  EnableKeyboardClicks (on by default).
- Preferences: a "Choose default sound" listbox under the cue checklist - it lists the
  selected cue's variants, arrowing it previews each sound and makes it that cue's
  default. Plus a "Play keyboard clicks when typing into any edit field" checkbox.
- KeyClickService.cs: an app-wide WM_CHAR message filter + a low-latency NAudio mixer.
  Typing into any edit field plays a random key click (key 1..N.wav); password fields
  also play passkey.wav at the same instant. On/off live from the Preferences toggle.
  Inert if the sounds are missing or the device won't open; never consumes the keystroke.
- csproj: ship every sounds\*.wav via a wildcard (variants, key clicks, passkey, future
  additions) instead of stale per-file canonical names.
- Tests: resource checks (self-test + run-tests.ps1) now verify each cue has >=1 variant
  and that key 1.wav / passkey.wav are present. Accessibility audit still green with the
  new Preferences controls (Alt+D, Alt+K - no mnemonic clashes).
- Manual: variant picker, keyboard clicks, and the new sound-file naming documented.

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

src/RemSound.App/SelfTest.cs

@@ -301,10 +301,16 @@ internal static class SelfTest
         // 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)");
+        // Cues ship as numbered variants ("connect 1.wav", ...); each required cue must have at
+        // least one variant present.
         foreach (var cue in new[] { "connect.wav", "disconnect.wav", "start up.wav" })
         {
-            Check(File.Exists(Path.Combine(soundsDir, cue)), $"cue sound '{cue}' must be present (was the shipped sounds\\ folder empty?)");
+            Check(CueSounds.Variants(cue).Count > 0,
+                $"no sound variant present for the '{Path.GetFileNameWithoutExtension(cue)}' cue (was the shipped 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");
+        Check(File.Exists(Path.Combine(soundsDir, "passkey.wav")), "password 'passkey.wav' must be present");
 
         // Native Opus (Concentus.Native) keeps the encoder off the allocation-heavy managed fallback.
         var nativeOpus = Path.Combine(root, "runtimes", "win-x64", "native", "opus.dll");