feat: add pre-built compression and code splitting for frontend assets

[Sumeet213]

Description:

Addresses #2340

• Pre-build gzip and brotli compressed assets at build time
• Serve pre-compressed files directly (no runtime CPU overhead)
• Split vendor libraries into cacheable chunks

Initial load reduced from ~1.07MB to ~850KB (brotli).

---

Summary by cubic

Pre-built gzip and Brotli assets and vendor code splitting to reduce initial load and remove runtime compression overhead, addressing #2340. Initial load drops from ~1.07MB to ~850KB.

• New Features

  • Backend serves .br/.gz files when supported (sets Content-Encoding and Vary).
  • Build-time compression via vite-plugin-compression for gzip and Brotli (keeps original files).
  • Manual code splitting for vendor chunks (react, UI, markdown, utils) to improve caching.

• Dependencies

  • Added vite-plugin-compression ^0.5.1.

^{Written for commit 7554526. Summary will update automatically on new commits.}

[cubic-dev-ai]

No issues found across 4 files