fix: enable gzip for static frontend assets

[bugkeep]

Summary

Enable gzip compression for embedded and disk-backed frontend assets.

CasOS currently serves the main JavaScript bundle uncompressed. On remote, low-resource machines, this makes the first browser load transfer several megabytes of JavaScript before the UI can start.

This change:

• Enables Beego gzip support.
• Compresses .js, .css, and .html assets when the client accepts gzip.
• Adds Vary: Accept-Encoding.
• Preserves the existing cache policy for hashed assets.
• Keeps API, Kubernetes proxy, and ACME challenge responses unchanged.
• Leaves HEAD and Range requests uncompressed.
• Preserves conditional requests, including 304 Not Modified.

No new dependency is introduced.

Validation

• go test -v ./routers -run '^TestStaticFilter' -count=1
• go test . -run '^$'
• gofmt
• git diff --check

All checks pass.