fix: handle sendFile misses over http

[cs50victor]

Summary

• add a shared sendFile error classifier so file-serving misses return explicit HTTP responses instead of falling through to Express default error handling
• route the oo-editors sendFile endpoints through that helper, including media, converted files, desktop stub assets, wasm redirects, fonts, and the service worker
• add unit coverage for the sendFile error classification helper

Why

Handled file misses should produce stable HTTP 404/500 responses without printing default Express stack traces to stderr. This keeps request-level file misses as HTTP behavior instead of process-level error noise.

Verification

• bun test tests/server-utils.test.js
• node --check server.js
• node --check server-utils.js