Handle all ignored errors

[Copilot]

Four error returns were being silently discarded with _ or not checked at all.

Changes

• main.go: Check http.ListenAndServe error, exit with code 1 on failure
• handlers.go: Log errors from w.Write, filepath.Rel, and cmd.Run using log.Printf
  • For filepath.Rel failures, fall back to absolute path

// Before_, _=w.Write(diffsHTML)
// After if_, err:=w.Write(diffsHTML); err!=nil {
log.Printf("Failed to write HTML response: %v", err)
}

Added test coverage for non-git directory handling to exercise error paths.

Original prompt

Please make sure no errors are swallowed.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.