Serve a proper 404 error page for missing static files

[bauerpawel]

Description

Requests for a static asset that doesn't exist (e.g. a stale/mistyped JS or CSS path) previously fell through http.FileServer to the plain stdlib 404 response instead of Gokapi's own styled error page.

Intercepts the http.FileServer response via a small http.ResponseWriter wrapper: if it would write a 404, the cache headers that were speculatively set for the (assumed-existing) file are removed and the request is redirected to Gokapi's existing generic error page (errorHandling.RedirectGenericErrorPage with TypeFileNotFound) instead.

Type of Change

• New feature (non-breaking change which adds functionality)

Technical Details

• Database changes: No
• Storage backend affected: No
• Usage of AI: Yes — developed with Claude Code as a pair-programming assistant. I reviewed and tested all changes before submitting.

How Has This Been Tested?

• Unit Tests: go test ./internal/webserver/... --tags=test,awsmock and full go test ./... --tags=test,awsmock suite pass; added TestStaticFileNotFound covering the new redirect behavior.
• Environment: Windows 11

Checklist

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation. (n/a — internal error-page behavior, no user-facing docs affected)
• My changes generate no new warnings.

🤖 Generated with Claude Code

https://claude.ai/code/session_0168YUwGEzEHTqd9CwzXE53d