fix(ubuntu): install dsearch command and user service

[kmf]

Summary

• add the dsearch command alias expected by DankMaterialShell
• ship dsearch.service as a systemd user unit
• run the packaged binary from /usr/bin/dsearch instead of the source-install path /usr/local/bin/dsearch

Root cause

The Ubuntu package installed only /usr/bin/danksearch, while DMS checks for and invokes dsearch. The DMS installer also attempts to enable dsearch.service, but the package did not ship that unit. Copying the upstream unit manually still failed because it referenced /usr/local/bin/dsearch.

This left file search unavailable in the DMS launcher even though the danksearch package was installed.

Impact

After rebuilding the package, DMS can detect dsearch, and its installer can enable and start the indexing service for the user.

Validation

• git diff --check
• systemd-analyze verify --man=no distro/ubuntu/danksearch/debian/dsearch.service
• dpkg-parsechangelog -l distro/ubuntu/danksearch/debian/changelog

Fixes AvengeMedia/danksearch#26