fix: use self-hosted dashboard URL in auth config error message

[majiayu000]

Fixes #56

Summary

When handlePushConfigError() encounters an AuthConfigMissingEnvironmentVariable error for self-hosted deployments, it falls through to a generic message without a dashboard link. This is because deploymentName is null for self-hosted setups (no deploymentFields).

This adds an else if branch that detects self-hosted mode via the CONVEX_SELF_HOSTED_URL env var and constructs the dashboard URL using hostname:6791 (the default self-hosted dashboard port from docker-compose.yml) with the path /settings/environment-variables?var=VAR_NAME.

Test plan

• All existing config.test.ts tests pass (14/14)
• Manual verification: set CONVEX_SELF_HOSTED_URL=http://localhost:3210 and trigger the AuthConfigMissingEnvironmentVariable error path — output shows http://localhost:6791/settings/environment-variables?var=VAR_NAME instead of dashboard.convex.dev

[ianmacartney]

Do you prefer the dashboard over npx convex env set ? Another way we could improve this would be to make the env set string include the name of the variable to set.
My only hesitation here is the hard-coding of the port, since you likely wouldn't have that port exposed for anything except when using localhost.

[majiayu000]

Thanks for the feedback @ianmacartney! You're right on both points — the hard-coded port 6791 would only be valid for localhost setups and isn't reliably exposed otherwise. I've updated the fix to instead include the variable name directly in the npx convex env set suggestion (e.g. npx convex env set AUTH_SECRET <value>), which works for all deployment types including self-hosted. The dashboard link for cloud deployments (when deploymentName is set) is unchanged.