T1.7: Create src/platform/desktop; move desktop integration

Move desktop_linux.go and desktop_other.go from src/core to the new
src/platform/desktop package. Update src/gui to import and use the
new package location.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/platform/desktop/desktop_other.go

@@ -0,0 +1,7 @@
+//go:build !linux
+
+package desktop
+
+func InstallDesktopIntegration(appID string, executablePath string, icon []byte) (string, error) {
+	return "", nil
+}