refactor: update sample pack and workspace schemas for slug uniqueness

Commit notes

- Modified samplePacks schema to enforce unique slugs within projects and for users without projects. - Updated sample pack routes to validate slug uniqueness based on project context during creation and updates. - Removed slug from workspace schema as it is no longer required. - Adjusted workspace setup logic to simplify slug generation for new projects. - Updated frontend to use workspace IDs instead of slugs for better consistency.