Add global search to navbar

[william08190]

Closes #3

Summary

• Add an authenticated /api/v1/search/ endpoint that searches records visible to the current user
• Return item results for normal users and include user results only for superusers
• Keep page matches in the navbar while loading item/user records from the backend search API
• Add backend route coverage for item visibility and superuser-only user results

Verification

• npm run build
• npx biome check --no-errors-on-unmatched --files-ignore-unknown=true src/components/Common/GlobalSearch.tsx src/components/Common/Navbar.tsx src/client/models.ts src/client/services.ts
• python3 -m compileall backend/app
• .venv/bin/ruff check app/api/routes/search.py app/api/main.py app/models.py app/tests/api/routes/test_search.py
• .venv/bin/mypy app/api/routes/search.py app/models.py app/tests/api/routes/test_search.py
• .venv/bin/pytest app/tests/api/routes/test_search.py
• git diff --check origin/main...HEAD