refactor(desktop): split backend and frontend modules

[tonyfettes]

Summary

• keep openseek_desktop target-neutral and let it own protocol, commands, uri, and file-search packages directly
• move the native executable, internal packages, tunnel, and packagers into openseek_desktop/backend; make both backend and frontend depend on the root module
• update artifact discovery, packaging paths, CI workflows, workspace membership, and documentation for the three-module layout

Testing

• just check
• just build
• just test (native 3379/3379, JS 3158/3158, cram 28/28)
• moon info
• git diff --check