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/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>4.4</Version>
+    <Version>4.5</Version>
   </PropertyGroup>
 
   <ItemGroup>