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/AboutDialog.cs

@@ -1351,7 +1351,7 @@ internal sealed class AboutDialog : Form
 
         var closeButton = new Button
         {
-            Text = "Close",
+            Text = "&Close",
             AutoSize = true,
             DialogResult = DialogResult.OK,
             TabIndex = 1,