thv llm setup: gateway URL not validated for HTTPS scheme (inconsistent with config set)

[yrobla]

Problem

thv llm config set --gateway-url correctly validates that the URL uses the https scheme:

if u.Scheme != "https" {
    return fmt.Errorf("gateway URL must use HTTPS: %s", llmConfigGatewayURL)
}

However, thv llm setup --gateway-url calls applySetupFlags, which writes the URL directly to config without any scheme validation. This means a user can accidentally configure an http:// gateway via setup, then hit a confusing error later at proxy creation time (inside llm.NewProxy).

Expected behavior

applySetupFlags (or newLLMSetupCmd's RunE) should apply the same HTTPS validation as llmConfigSetFunc when --gateway-url is changed.

Acceptance Criteria

• thv llm setup --gateway-url http://... returns an error: "gateway URL must use HTTPS"
• Validation is consistent between thv llm config set and thv llm setup

References

• cmd/thv/app/llm.go — llmConfigSetFunc (has validation) vs applySetupFlags (no validation)

[yrobla]

Implementation plan

This bug fix will be included in PR 1 alongside #5123, #5124, and #5129.

Change: Apply the same HTTPS validation already present in llmConfigSetFunc to applySetupFlags / the setup command's RunE, so both entry points reject non-HTTPS gateway URLs consistently.

[yrobla]

Covered by #5123 — implemented in the same PR.