fix(web): serve JavaScript assets with module-safe MIME type

[Sisyphbaous-DT-Project]

Related Issue

Resolve #2074

Description

This PR fixes a Web UI startup failure on Windows where JavaScript assets can be served with text/plain instead of a browser-safe JavaScript MIME type.

On some Windows environments, Python's MIME type detection can resolve .js files to text/plain. Since the Web UI loads Vite-built assets as module scripts, browsers reject those responses with strict MIME checking before the page can finish loading.

The fix is intentionally scoped to the /web static file path:

• add a small StaticFiles subclass for the Kimi Web UI
• preserve the existing Starlette static-file behavior
• override Content-Type only for served .js assets
• avoid changing CLI startup, auth/CORS, session worker logic, frontend build output, or global MIME mappings

A regression test simulates the problematic .js -> text/plain MIME mapping and verifies that the served asset still receives a module-safe JavaScript MIME type.

Tests

• python -m pytest tests/web/test_static_mime.py -vv
• python -m pytest tests/web -vv

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog. (N/A — targeted Web UI MIME bugfix)
• I have run make gen-docs to update the user documentation. (N/A — no user-facing documentation changes)

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.