Upgrade Angular to v21, resolve vulnerabilities and configure devcontainer

[sis0k0]

Summary

• upgrade client from Angular 17 to Angular 21 (CLI/framework/material/cdk/ssr)
• apply Angular migrations and SSR/bootstrap updates
• remediate npm vulnerabilities in client and server lockfiles
• add/standardize repository crawlability assets (README, AGENTS.md, EDD.md, CI, metadata)
• add deterministic MongoDB seed script and wire npm commands
• improve devcontainer/Codespaces config and MongoDB readiness checks

Validation

• npm run build:server
• npm run build:client
• npm run test:server
• npm audit --json (client/server show 0 vulnerabilities)