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/Program.cs

@@ -130,6 +130,13 @@ internal static class Program
         // is per-thread and modifier-aware: bare F1 only, so Shift/Ctrl/Alt+F1 stay free.
         HelpLauncher.Install();
 
+        // Audible typing feedback: a soft click on each keystroke in any edit field, plus a distinct
+        // passkey sound on password fields. Machine-wide toggle (on by default). Installed app-wide
+        // here - after the single-instance guard, before the profile picker - so it works on the
+        // picker and every dialog. Best-effort: inert if the click sounds can't load.
+        KeyClickService.Initialize(AppConfig.Load().EnableKeyboardClicks);
+        Application.ApplicationExit += (_, _) => KeyClickService.Shutdown();
+
         // One-time "your settings moved" notice — only the launch that actually relocated files
         // shows it (idempotent migration ⇒ MovedAnything is false on every later launch). Shown
         // here, after the guard and before the profile picker, so the user reads it once up front.
@@ -436,10 +443,11 @@ internal static class Program
             var cfg = AppConfig.Load();
             if (!cfg.EnableStartupCue) return;
             var custom = cfg.StartupCueCustomPath;
+            // Custom override wins; otherwise the chosen default variant ("start up 1.wav" etc).
             var path = !string.IsNullOrWhiteSpace(custom) && File.Exists(custom)
                 ? custom
-                : Path.Combine(AppConfig.SoundsDirectory, "start up.wav");
-            if (!File.Exists(path)) return;
+                : CueSounds.ResolveDefaultPath(MainForm.CueId.Startup, "start up.wav", cfg);
+            if (path is null || !File.Exists(path)) return;
             new CuePlayer(path).Play();
         }
         catch { /* a startup cue must never disturb startup */ }