fix(web): serve requests concurrently

[teddyli18000]

Summary

• replace the single-threaded codex-web request loop with a fixed 8-worker pool
• share the HTTP server and Tokio runtime across workers without changing request handling behavior
• add a regression test proving a blocked request does not prevent another request from being served

Validation

• rustfmt --edition 2021 --check src-tauri/src/web.rs
• cargo test --manifest-path src-tauri/Cargo.toml — 45 passed, 0 failed
• pnpm build

Fixes #146