ci: Add conditional Remote DroidGuard Server build job

[Copilot]

The remote-droidguard-server module only exists on certain branches, so running its build job unconditionally would fail on branches like master where the folder is absent.

Changes

• New job build-remote-droidguard in build.yml that builds and uploads RemoteDroidGuard-Server-debug.apk
• Folder existence gate: immediately after checkout, checks for remote-droidguard-server/ and writes to GITHUB_OUTPUT; all subsequent steps (Java, Gradle setup, assemble, upload) are conditioned on steps.check_folder.outputs.exists == 'true', so the job exits cleanly on branches without the module

- name: "Check if remote-droidguard-server folder exists"
  id: check_folder
  run: |
    if [ -d "remote-droidguard-server" ]; then
      echo "exists=true" >> "$GITHUB_OUTPUT"
    else
      echo "exists=false" >> "$GITHUB_OUTPUT"
      echo "remote-droidguard-server folder not found; skipping build."
    fi

- name: "Assemble debug APK"
  if: steps.check_folder.outputs.exists == 'true'
  run: "./gradlew :remote-droidguard-server:assembleDebug"