refactor: adopt baby-nas Makefile style with inline packaging targets

- Simplify build targets: frontend, build, build-go, build-all, run, package
- Remove scripts/ for packaging (inline in Makefile)
- Add etc/syncserver/config.yaml and lib/systemd/system/ at project root
- build-all produces both amd64 and arm64 binaries
- package target builds .deb inline without external scripts
- VERSION from git tags, commit hash injected via ldflags

etc/syncserver/config.yaml

@@ -0,0 +1,9 @@
+data_dir: /var/lib/syncserver
+addr: :8080
+
+auth:
+  jwt_secret: ""
+  jwt_expiry_hours: 24
+
+scheduler:
+  timezone: UTC