v5.1: install RemSound as a proper Windows app, plus smaller sounds

New Options -> Install / Uninstall RemSound on this PC: a per-user self-installer
(%LOCALAPPDATA%\Programs\RemSound, no admin) with optional desktop + Start-menu
shortcuts, login auto-start (reuses StartupAutoStart), Windows Installed-apps
registration, and copy-across of profiles+config, recordings and logs. Install
state is decided by a marker file, not a folder-path guess; the post-install
relaunch hands over foreground via AllowSetForegroundWindow so the installed copy
comes to the front; uninstall uses a batch remover (no PowerShell) and confirms
with two independent tick-boxes. All new dialogs use the house accessible controls
(AccessibleCheckBox, Theme.Heading).

Also: iOS (TestFlight) companion link alongside Android in README + manual;
slimmed-down default cue WAVs; About/RELEASE_NOTES/manual updated; version -> 5.1.

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

src/RemSound.App/RemSound.App.csproj

@@ -18,7 +18,7 @@
          tag_name on the latest GitHub release; bump it on every public release. The
          AssemblyVersion / FileVersion default to this value, and Assembly.GetName().Version
          is what the About dialog and the updater both read. -->
-    <Version>5.0</Version>
+    <Version>5.1</Version>
   </PropertyGroup>
 
   <ItemGroup>