Release v4.4: keyboard shortcuts are now machine-wide, not per-profile

Fixes issue #14: shortcuts were stored on each Profile, so one set on profile A
didn't apply on profile B and seemed to vanish on switch. They now live in
AppConfig (one set shared by every profile). RemSoundSettingsStore's Load*/Save*
hotkey methods re-point to AppConfig (callers unchanged); the per-profile cache
fields, profile load/save plumbing, and the HotkeySetting helper class are removed.
Profile's HotkeyRecord fields stay only for back-compat deserialization.

On upgrade, shortcuts reset to defaults (there's no single correct set to carry over
since profiles could hold different/partial sets). A one-time startup notice tells
upgraders to re-set them; fresh installs are silently marked done (nothing to reset).

Manual, About changelog and RELEASE_NOTES updated; csproj <Version> 4.4.

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

src/RemSound.App/RemSound.App.csproj

@@ -14,7 +14,7 @@
          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>4.3.1</Version>
+    <Version>4.4</Version>
   </PropertyGroup>
 
   <ItemGroup>