Add user management, change log and customer archive

- Multi-user auth with username/password (replaces single password)
- Default admin account created automatically (admin/cuitunet2024)
- User CRUD with admin/user roles (only admin can manage users)
- All customer changes logged with timestamp, user and details
- Customer deletion now archives instead of permanently removing
- Archive view with restore and permanent delete options
- Tab navigation: Asiakkaat, Arkisto, Muutosloki, Käyttäjät
- Protect users.json, changelog.json and archive.json in .gitignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,6 @@
 data/customers.json
+data/users.json
+data/changelog.json
+data/archive.json
 data/backups/
 data/files/