test(frontend): add page.server tests for admin pages

[fpindej]

Problem

Eight admin +page.server.ts files have no tests:

• (app)/admin/users/+page.server.ts - pagination, search params, PII masking
• (app)/admin/users/[id]/+page.server.ts - user detail load, error handling
• (app)/admin/roles/+page.server.ts - roles list load
• (app)/admin/roles/[id]/+page.server.ts - role detail, permissions load
• (app)/admin/jobs/+page.server.ts - jobs list load
• (app)/admin/jobs/[jobId]/+page.server.ts - job detail load
• (app)/admin/oauth-providers/+page.server.ts - providers list load
• (app)/+page.server.ts - root app redirect

The admin/layout.server.test.ts already exists and demonstrates the pattern.

Reference

• src/frontend/src/routes/(app)/admin/layout.server.test.ts

Scope

Prioritize users and roles pages (most branching logic). Jobs and oauth-providers are simpler loads.