fix: handle unchecked error return values

[thrawn01]

Purpose

Fix errcheck linter warnings that were causing CI failures after merging PR #21.

Implementation

• Added error handling for r.ParseForm() in the /set HTTP handler
• Added explicit ignore (_, _ =) for w.Write() calls in HTTP handlers where errors cannot be meaningfully handled
• Wrapped deferred Close() calls in anonymous functions with explicit ignore to satisfy errcheck
• Added proper error checking for sink.SetBytes() in tests
• Changed dest.SetString() to return its error value in the getter function