Fix UPnP close-hang, and add mnemonics to every dialog's buttons (Andre)

UPnP close hang:
- RouterPortMapper.Stop()/Dispose() does a SYNCHRONOUS DeletePortMap call to the router
  (plus StopDiscovery). When the router is slow or unresponsive that blocks — and it ran on
  the UI thread in FormClosing, so enabling UPnP could make RemSound impossible to close
  (Andre, this morning; his logs show UPnP teardown taking several seconds even on a good
  run). Moved the router teardown into the same bounded background-task pattern already used
  for the ASIO audio dispose: UPnP + audio now tear down off the UI thread, in parallel, under
  one 3s cap. Anything unfinished is reclaimed on process exit, so the window always closes.

Mnemonics on dialog buttons:
- Andre flagged the "RemSound is already running" dialog (single-instance) having no shortcut
  keys. Its three TaskDialog buttons had no & mnemonics; added Alt+S / Alt+F / Alt+C.
- Swept every dialog. Custom TaskDialog buttons and Form OK/Cancel/Close buttons that lacked
  mnemonics now have them: single-instance (Switch/Force/Cancel), save-onto-read-only
  (Save/Cancel), Add EQ band, Change/Manage profile password, Quick profile switch (Close),
  Rename peer (OK), About (Close). Cancel stays on Esc in the few dialogs where Alt+C is
  already taken (Rename peer's Clear, Service profile's peers list) — by design, not a miss.
- The rest already had mnemonics (Keyboard-shortcut import, Update-install notice, Profile
  selection, Recording settings). The dialog-accessibility gate (mnemonic uniqueness + names)
  stays green, so nothing collides.

Gate 46/46.

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

src/RemSound.App/MainForm.cs

@@ -1434,7 +1434,10 @@ public sealed class MainForm : Form
             try { processSelfMeter.Dispose(); } catch { }
             try { deviceChangeNotifier?.Dispose(); } catch { }
             try { powerResumeHandler?.Dispose(); } catch { }
-            try { routerPortMapper?.Dispose(); } catch { }
+            // NOTE: routerPortMapper.Dispose() is NOT called here — it's moved into the bounded
+            // background teardown below. Its Stop() does a SYNCHRONOUS DeletePortMap call to the router,
+            // which blocks (or hangs) when the router is slow/unresponsive; on the UI thread that froze the
+            // close, so enabling UPnP made RemSound impossible to shut (Andre, 2026-07-18).
             // Reverse every Win32 lever PerformanceMode applied. The kernel would clean
             // these up on process exit anyway, but doing it explicitly releases the power
             // request handle and matches our timeBeginPeriod with a timeEndPeriod.
@@ -1449,14 +1452,22 @@ public sealed class MainForm : Form
             // the form-close path run; the OS reclaims any audio resources on process exit.
             // Worst case the user sees a brief tray-icon stutter; before this they saw a
             // ~16 s frozen window before the form went away.
-            var audioDispose = Task.Run(() =>
+            // The UPnP router teardown (DeletePortMap — a synchronous call to a possibly-unresponsive
+            // router) belongs here too: like the ASIO dispose it can block for many seconds, and on the UI
+            // thread it froze the close. Run both OFF the UI thread, in parallel, under one hard timeout.
+            var slowTeardown = Task.Run(() =>
             {
-                try { sender.Dispose(); } catch { /* ignore */ }
-                try { receiver.Dispose(); } catch { /* ignore */ }
+                var upnp = Task.Run(() => { try { routerPortMapper?.Dispose(); } catch { /* ignore */ } });
+                var audio = Task.Run(() =>
+                {
+                    try { sender.Dispose(); } catch { /* ignore */ }
+                    try { receiver.Dispose(); } catch { /* ignore */ }
+                });
+                try { Task.WaitAll(upnp, audio); } catch { /* ignore */ }
             });
-            if (!audioDispose.Wait(TimeSpan.FromSeconds(2)))
+            if (!slowTeardown.Wait(TimeSpan.FromSeconds(3)))
             {
-                try { logFile.Event("close: audio dispose taking >2s; letting process exit reclaim"); } catch { }
+                try { logFile.Event("close: UPnP/audio teardown taking >3s; letting process exit reclaim"); } catch { }
             }
 
             hotkeyController.Dispose();
@@ -2812,8 +2823,8 @@ public sealed class MainForm : Form
     private bool ShowSaveOnReadOnlyWarningDialog()
     {
         var verification = new TaskDialogVerificationCheckBox("Do not show me this message again");
-        var saveButton = new TaskDialogButton("Save anyway");
-        var cancelButton = new TaskDialogButton("Cancel") { AllowCloseDialog = true };
+        var saveButton = new TaskDialogButton("&Save anyway");
+        var cancelButton = new TaskDialogButton("&Cancel") { AllowCloseDialog = true };
         var page = new TaskDialogPage
         {
             Caption = AppName,