Fix incomplete path traversal check in FileResponse (nwu + nwu2)

[Copilot]

The .. traversal guard only checked ../ (forward slash), missing ..\ (Windows backslash separator). While the canonical path comparison is the authoritative defense, the explicit check should be comprehensive on all platforms.

Changes

• src/com/nwu/httpd/responses/FileResponse.java and src/com/nwu2/httpd/responses/FileResponse.java
  • Added || relativeUri.indexOf(".." + File.separator) >= 0 to block backslash-based traversal sequences
  • Updated error message from "Won't serve ../ for security reasons." → "Won't serve .. paths for security reasons." to be platform-agnostic

// Before
if (relativeUri.startsWith("..") || relativeUri.endsWith("..")
        || relativeUri.indexOf("../") >= 0)

// After
if (relativeUri.startsWith("..") || relativeUri.endsWith("..")
        || relativeUri.indexOf("../") >= 0
        || relativeUri.indexOf(".." + File.separator) >= 0)

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