Phase 3 polish: admin fixes, model suggestions, test env shim

[caioopra]

Summary

Polish pass on top of the Phase 3 admin console. Mostly bug fixes plus a small UX addition and a test-environment fix.

• fix(admin): dashboard was reading a nonexistent active_provider setting key; now reads the real llm_default_provider so the Active Provider card shows the correct value instead of —
• fix: "Chat Feature" and "LLM Configuration" headings on the Providers page were rendering as black-on-dark and unreadable; added a body color fallback in index.css and inlined the heading colors
• feat(admin): provider model fields now show a curated <datalist> of common Gemini/Claude model IDs while still allowing free-text entry
• test(frontend): the full suite (267 tests) was failing because vitest 4 + jsdom 25 didn't give Zustand's persist middleware a working localStorage; added a Map-backed storage shim in src/test/setup.js
• docs: added docs/testing_phase3.md manual QA guide; dropped stale test-DB references from infra docs and docker-compose

Test plan

• npm test — 31 files / 267 tests pass
• cargo test — backend suite
• cargo clippy -- -D warnings
• Manual: admin dashboard cards render correct values
• Manual: Providers page headings readable, model dropdowns show suggestions
• Manual: kill-switch, rate limit, audit log walkthrough

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