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/MainForm.cs

@@ -6486,14 +6486,16 @@ public sealed class MainForm : Form
             }
             else
             {
-                var defaultPath = Path.Combine(AppConfig.SoundsDirectory, defaultFileName);
-                if (File.Exists(defaultPath))
+                // The cue ships as numbered variants ("connect 1.wav", "connect 2.wav", ...);
+                // resolve the machine-wide chosen default (or the first variant) for this cue.
+                var defaultPath = CueSounds.ResolveDefaultPath(cueId, defaultFileName, AppConfig.Load());
+                if (defaultPath is not null && File.Exists(defaultPath))
                 {
                     path = defaultPath;
                 }
                 else
                 {
-                    logFile.Event($"cue sound '{cueId}': default missing ({defaultPath}) and no custom override set — cue will be silent");
+                    logFile.Event($"cue sound '{cueId}': no default variant found in sounds\\ and no custom override set — cue will be silent");
                     return;
                 }
             }