fix: classify request errors and give clear user-facing messages (#319)

[surajpatidar1]

Fixes #319

• Adds error classification (INVALID_URL, NETWORK_ERROR, TIMEOUT, CANCELLED, UNKNOWN) with clear user-facing messages
• Validates URL before fetch instead of letting a raw TypeError bubble up
• Adds a 30s request timeout so hung requests don't fail silently forever
• Reworked cancel logic to support multiple concurrent requests safely
• Tested manually: invalid URL, unreachable host, and successful request all return correct codes/messages