Release v4.5: import shortcuts on pre-4.4 upgrade + clearer shortcuts dialog

- Pre-v4.4 upgraders are offered a one-time dialog to copy their keyboard shortcuts
  from one of their profiles (still readable in the profile files) instead of being
  reset. Users who already went through v4.4's reset are deliberately NOT re-offered
  (gated on KeyboardShortcutsGlobalNoticeShown). New KeyboardShortcutImportDialog +
  AppConfig.KeyboardShortcutsImportOffered + MainFormHotkeyController.ReloadAndReRegisterAll.
- Keyboard shortcuts dialog: new "Clear this shortcut" button; Delete inside the
  capture box leaves a shortcut unassigned.
- Relabelled the three RemSound-app remote rows to "Send remote RemSound volume/..."
  to distinguish them from the Windows-global ones.
- Manual (readme.html + MANUAL.md), About changelog, RELEASE_NOTES updated; version 4.5.

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

src/RemSound.App/AboutDialog.cs

@@ -20,6 +20,26 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v4.5
+
+        Keyboard shortcuts get easier to manage.
+
+        If you're updating from before version 4.4,
+        RemSound now offers to bring your shortcuts
+        across from one of your profiles instead of
+        resetting them — pick the profile you set them
+        up in, or start fresh.
+
+        The Keyboard shortcuts dialog has a new "Clear
+        this shortcut" button, and when you're setting a
+        shortcut you can press Delete to leave it
+        unassigned.
+
+        And the three remote-control rows now name
+        RemSound — "Send remote RemSound volume", and so
+        on — so they're easy to tell apart from the
+        Windows-volume ones.
+
         RemSound v4.4
 
         Keyboard shortcuts are now shared across all