Add global search

[Boo4S]

Closes #3

Summary

• Add an authenticated /api/v1/search/ endpoint.
• Search visible items and users while preserving existing permission boundaries.
• Add a global search input to the navbar with result dropdowns.

Validation

• npm run build
• uv run --python 3.10 ruff check app/api/routes/search.py app/api/main.py app/models.py app/tests/api/routes/test_search.py
• ./node_modules/.bin/biome check --apply-unsafe --no-errors-on-unmatched --files-ignore-unknown=true src

Backend pytest was attempted, but local Postgres authentication failed for postgres@localhost:5432.