chore: improve local bootstrap and add rich mock seed flow

[bychrisr]

Summary

• improve local dev bootstrap scripts to reduce environment startup friction
• add richer mock database seeding and reset scripts for manual QA flows
• fix CORS defaults for Vite local ports and improve API unavailable handling in web/admin
• add favicon assets for admin/web and stricter Vite port behavior

What changed

• root scripts: dev, dev:mock, dev:mock:web, dev:mock:all
• API scripts: db:seed:mock, db:reset:mock
• API seed refactor to support custom seed datasets
• new mock dataset covering public/private events, release windows, materials, and quiz submission-ready UUIDs
• docs updates for bootstrap and seed usage

Validation

• npm run db:reset:mock --workspace @flix/api
• npm run test --workspace @flix/api -- tests/integration/db-bootstrap.test.js
• npm run dev:mock (startup verified)
• CORS preflight verified for http://localhost:5173