feat: add fluxer upstream source and self-hosting documentation

- Clone of github.com/fluxerapp/fluxer (official upstream)
- SELF_HOSTING.md: full VM rebuild procedure, architecture overview,
  service reference, step-by-step setup, troubleshooting, seattle reference
- dev/.env.example: all env vars with secrets redacted and generation instructions
- dev/livekit.yaml: LiveKit config template with placeholder keys
- fluxer-seattle/: existing seattle deployment setup scripts

fluxer/fluxer_desktop/packaging/linux/app.fluxer.Fluxer.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Name=Fluxer
+Comment=Instant messaging and VoIP application
+Exec=app.fluxer.Fluxer %U
+Terminal=false
+Type=Application
+Icon=app.fluxer.Fluxer
+Categories=Network;InstantMessaging;
+StartupWMClass=app.fluxer.Fluxer