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/AboutDialog.cs

@@ -20,6 +20,20 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v4.4
+
+        Keyboard shortcuts are now shared across all
+        your profiles, instead of being saved separately
+        in each one. Set a shortcut once and it works on
+        every profile, and stays put when you switch —
+        which is what people asked for.
+
+        Because of this change, your shortcuts have gone
+        back to their defaults. If you'd set up any of
+        your own, please set them again in Options →
+        Keyboard shortcuts (Ctrl+K). You only need to do
+        this once.
+
         RemSound v4.3.1
 
         A gentler update sound. The cue that plays when