feat(web): rewrite frontend with Vite

[ieraasyl]

Summary

• replace the shipped SPA with the Vite 8, React 19, and TypeScript 7 web app
• introduce typed API contracts, route-backed state, query boundaries, auth guards, and a smaller modular frontend architecture
• port and polish announcements, events, communities, opportunities, courses, planner, degree audit, SGotinish, profile, contacts, and public pages
• include the supporting backend security and reliability fixes, realistic development fixtures, and CI/deployment cutover

Verification

• pnpm format:check
• pnpm lint
• pnpm test
• pnpm typecheck
• pnpm build
• pnpm api:check

Handoff

• frontend/ remains unchanged as the temporary rollback fallback
• docs/web-parity-uat.md contains the remaining manual viewport, theme, reduced-motion, and workflow acceptance checks
• commit history is organized as one-line Conventional Commit subjects for review