feat: add project evaluator template gallery

[yfrigui2]

Summary

• add a browsable project evaluator template gallery backed by the existing classification evaluator configs
• present the gallery as a project-level full-page route with global navigation and an Evaluator gallery breadcrumb
• restore the evaluator-list empty state and add direct and dropdown entry points to the gallery
• keep template, blank LLM, and code evaluator creation flows URL-addressable over the gallery

Screenshots

Evaluator empty state with a direct gallery entry point:

Full-page evaluator gallery:

Test plan

• make typecheck-frontend
• make lint-frontend
• make test-frontend (210 files passed; 2,333 tests passed and 12 skipped)
• browser-tested the empty-state entry point, gallery route and breadcrumb, responsive layout, refresh persistence, and template/blank/code creation slideovers