feat(desktop): add Conveyor packaging for cross-platform distribution

Replace the 4-OS matrix desktop build in release.yml with a single
Conveyor runner on ubuntu-24.04 that cross-compiles all platform
packages (macOS, Windows, Linux) from one machine.

- Add Conveyor Gradle plugin (dev.hydraulic.conveyor v2.0)
- Add conveyor.conf with app metadata, icons, JVM modules, entitlements
- Add ci.conveyor.conf for CI overrides (production URL, Apple notarization)
- Add defaults.conf.example template for local signing keys
- Update release.yml: single Conveyor job replaces 4-OS matrix
- Add 4 optional secrets: CONVEYOR_SIGNING_KEY, APPLE_TEAM_ID,
  APPLE_ID, APPLE_APP_SPECIFIC_PASSWORD
- Add Conveyor Maven repo to settings.gradle.kts

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

desktop/defaults.conf.example

@@ -0,0 +1,11 @@
+// Local Conveyor defaults — copy to defaults.conf and fill in your values.
+// This file is checked in as a template; defaults.conf is gitignored.
+conveyor.billing-email = "you@example.com"
+
+// Uncomment for real code signing:
+// app.signing-key = "<your-root-key>"
+// app.mac.notarization {
+//     team-id = "YOUR_TEAM_ID"
+//     apple-id = "you@example.com"
+//     app-specific-password = ${env.APPLE_ASP}
+// }