Bump to v2.1.0: UPnP, read-only profile lock, sleep/hibernate audio fix

Headline features:
* Automatic router port opening (UPnP / NAT-PMP / PCP). Opt-in via
  Preferences; surfaces external address + carrier-grade NAT detection.
* Lock profile (read-only). New File-menu tick that makes a profile
  load-only — session changes don't persist, no save prompt on close.
  Unblocks unattended shutdowns (NVDA gone, remote dropped, hibernate)
  where the existing save prompt could deadlock.
* Check for updates on startup (default on) + brief countdown notice
  before silent updates install, so a launch-time update doesn't make
  the app silently vanish.
* "Cue sounds" -> "Audio cue sounds" label clarification.

Bug fixes:
* No sound after the computer wakes from sleep. PowerResumeHandler
  rebuilds the audio backend automatically on resume; brief
  "Reconnecting to audio driver" splash during the rebuild.
* Receiver audio silent after waking from hibernate. RefreshAudioDeviceLists
  now treats a transient ASIO probe failure (returns -1/-1 because the
  driver is mid-teardown / mid-reinit) as "retry next tick" instead of
  clearing the user's tick selection.

Diagnostic-only changes (gated on the existing Enable-logs checkbox,
zero cost when off):
* AudioStepProbe split into cross-buffer vs within-buffer maxes so log
  inspection can tell a real-content sharp transient apart from a
  pipeline-boundary glitch. Plumbed through every probe owner.
* New rxNetGapMs + gc0/gc1/gc2 delta columns in the diag log to split
  receive-side jitter into network-layer vs managed-runtime causes.

Files touched: RELEASE_NOTES.md + readme.html + 24 source files across
RemSound.Core / RemSound.Sender / RemSound.Receiver / RemSound.App.
Three new app files: PowerResumeHandler, RouterPortMapper,
UpdateInstallNoticeDialog.

Wire format and audio pipeline unchanged from v1.5 onward — v1.5
through v2.1 peers interoperate.

src/RemSound.Core/AppConfig.cs

@@ -49,6 +49,14 @@ public sealed class AppConfig
     /// itself is the user-visible confirmation, so a follow-up popup is redundant.</summary>
     public bool SaveProfileConfirmationSuppressed { get; set; }
 
+    /// <summary>True if the user has ticked "do not show me this message again" on the
+    /// "this profile is read-only, save was skipped" popup. Once ticked, Ctrl+S / File → Save
+    /// on a read-only profile silently does nothing instead of explaining why — the user
+    /// has acknowledged that they know it's a no-op. Machine-local (not per-profile) so the
+    /// preference sticks across profile switches; the prompt itself is the same wording on
+    /// any read-only profile so a single dismissal applies everywhere. 2026-05-22.</summary>
+    public bool SaveOnReadOnlyMessageSuppressed { get; set; }
+
     /// <summary>If true, RemSound minimises to the system tray immediately after the main
     /// window finishes loading. Lets the user "boot up the machine and have RemSound
     /// already running quietly". Default false.</summary>
@@ -83,6 +91,22 @@ public sealed class AppConfig
     /// RemSound. Default false — the user gets a confirmation dialog before each install.</summary>
     public bool SilentlyInstallUpdates { get; set; }
 
+    /// <summary>If true (the default), RemSound runs an update check shortly after launch in
+    /// addition to whatever <see cref="UpdateCheckFrequency"/> drives in the background. The
+    /// startup check is what catches users who quit and re-open the app within the polling
+    /// interval — without it they could miss an update for hours. Set to false to disable the
+    /// startup check; the periodic timer (if set) still runs.</summary>
+    public bool CheckForUpdatesOnStartup { get; set; } = true;
+
+    /// <summary>If true, RemSound tries to open the audio port (UDP 47830) on the local router
+    /// using UPnP / NAT-PMP / PCP, so peers on the public internet can reach this machine
+    /// without manual port forwarding. Default false — the toggle opt-in only, because some
+    /// networks (corporate, hostile shared) shouldn't have apps poking the router. When
+    /// successful, RemSound surfaces the external address in the Preferences dialog so the
+    /// user knows what to give peers. Falls back gracefully when the router doesn't support
+    /// UPnP — RemSound just doesn't open anything.</summary>
+    public bool UpnpEnabled { get; set; }
+
     /// <summary>UTC timestamp of the last successful update check. Used by the background
     /// update timer to space out polls across launches — if you set the frequency to
     /// "every 24 hours" and re-launch the app three times that day, it still hits the API