fix(setup): derive signin URL from api_url for self-hosted instances

[manumishra12]

Summary

Fixes #28 — when git-lrc is configured against a self-hosted LiveReview instance, the signin / "Re-authenticate" flow always redirected to the cloud hexmos.com domain instead of the self-hosted server. Self-hosted users saw not authenticated, and clicking Re-authenticate bounced them to hexmos.com even though review-on-commit requests were correctly going to their local instance.

Root cause

runHexmosLoginFlow already had the configured apiURL in scope (self-hosted-aware), but BuildSigninURL(callbackURL) ignored it and always parsed the hardcoded cloud constant HexmosSigninBase = "https://hexmos.com/signin". Both initial setup and the UI re-authenticate button share this code path, so both were affected.

Fix

Thread apiURL into BuildSigninURL and resolve the signin base via a new ResolveSigninBase(apiURL) with this precedence:

1. LRC_SIGNIN_URL env override, if set;
2. cloud default (hexmos.com/signin) when api_url is empty or the cloud URL — cloud behavior is unchanged;
3. otherwise <api_url>/signin, so a self-hosted instance signs in against itself.

Also relaxed the signin base validation to allow http (local self-hosted instances), still requiring a valid scheme and a non-empty host. The <api_url>/signin derivation mirrors the existing network/endpoints.go convention (strings.TrimSuffix(apiURL, "/") + path).

⚠️ Note for maintainers

The <api_url>/signin path is a reasonable assumption — I couldn't find a documented convention in the repo for where a self-hosted LiveReview serves its signin page. If that path is wrong, the LRC_SIGNIN_URL override means self-hosters aren't blocked, but please confirm the correct path so the derived default can be corrected if needed.

Testing

• go build ./..., go vet, gofmt -l all clean
• setup and internal/appui packages pass, including new BuildSigninURL tests covering: cloud default, empty api_url, self-hosted derivation, trailing-slash api_url, and the LRC_SIGNIN_URL override.