Debian support: verified build/packaging path + friend install guide

- setup-standalone.sh: falls back to system python3 -m venv + pip when
  uv isn't installed (Debian/Ubuntu path; verified on clean bookworm).
- dist/Dockerfile.debian: debian:bookworm build env for :app:packageDeb
  (needs fakeroot + /usr/share/desktop-directories — both baked in; the
  dir gap made jpackage's postinst fail with exit 3 on minimal systems).
- dist/make-release.sh: clean source tarball from HEAD.
- INSTALL-debian.md: end-to-end install for friends (source tree must
  stay on disk — the app spawns the engine from it via SHONAR_REPO).

Verified on real Debian 12 in Docker: :app:packageDeb builds
shonar-desktop_0.1.0-1_amd64.deb, apt installs it, setup-standalone.sh
provisions the engine (venv+pip path, model fetch, boot smoke test),
and the packaged binary launches under Xvfb.

.gitignore

@@ -8,3 +8,5 @@ backend/__pycache__/
 *.pyc
 .dev-restart-flag
 models/hf-cache/
+dist/*.deb
+dist/*.tar.gz