docs: expand README with setup, testing, and architecture guides

[Erij-Maherzia-BEN-BRAHIM]

Summary

Expands the admin panel README to improve onboarding for new contributors and developers evaluating the project.

Motivation: The previous README assumed a running LibreChat instance and only documented a minimal bun install && bun dev flow. It did not explain the BFF → Admin API architecture, Node.js version requirements, how to explore the UI without credentials, available npm scripts, or how to run tests.

Changes:

• Clarifies that the panel proxies the LibreChat Admin API (not a direct DB connection)
• Adds architecture diagram and prerequisites (Node ≥ 22.12)
• Documents two setup paths: mock backend (UI exploration) vs real LibreChat connection
• Adds scripts reference, testing section (Vitest + Playwright), and project structure overview
• Adds capability grants to the features list and ADMIN_PANEL_METRICS_SECRET to the env var table

No code behavior changes in this PR.

Change Type

Please delete any irrelevant options.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Translation update

Testing

Docs-only change. Verified manually:

1. Read through all new sections for accuracy against package.json, playwright.config.ts, and e2e/mock-backend.mjs
2. Confirmed mock-backend instructions match existing Playwright setup (VITE_API_BASE_URL=http://localhost:3081)
3. Confirmed Node version requirement matches project dependencies (Vite 8 / TanStack Start)

Test Configuration:

• N/A — documentation only

Checklist

Please delete any irrelevant options.

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes
• Any changes dependent on mine have been merged and published in downstream modules.
• A pull request for updating the documentation has been submitted.