Signer: Add explicit HTTP client timeouts and avoid http.DefaultClient

[viktorking7]

• Summary: Ensure robust networking by setting explicit HTTP client and transport timeouts; stop using http.DefaultClient when TLS is disabled.
• Changes:
  • Add net import.
  • Configure http.Client Timeout (30s).
  • Configure http.Transport: Dial timeout (10s), TLS handshake (10s), response header (20s), expect-continue (1s), idle (90s), max idle conns (100).
  • Preserve TLS settings (TLS 1.3 min, RootCAs, dynamic client cert via certman).
• Motivation: Prevent indefinite hangs and resource leakage under network failures or slow peers.
• Impact: Safer, more predictable network behavior; no functional API change intended.