Replace boolean parameters with options pattern (spaFallback, SetAllowPrivateIPs)

[intel352]

Problem

Two functions use ambiguous boolean parameters:

• NewStaticFileServer(root string, spaFallback bool) — call sites read as NewStaticFileServer("/", true) with no context
• SetAllowPrivateIPs(allow bool) — two booleans in one method

Fix Direction

Use the functional options pattern or separate methods:

• NewStaticFileServer(root string, opts ...StaticFileServerOption) with WithSPAFallback() option
• Replace SetAllowPrivateIPs(bool) with AllowPrivateIPs() / DisallowPrivateIPs() or an options struct

Context

See docs/DEFERRED_ISSUES.md — Code Smells section.