Support self-hosted Git providers with custom server URLs

[Tranthanh98]

Problem

Commit+ currently supports hosted Git providers such as GitHub.com, but users may also work primarily with self-hosted / on-prem Git servers using custom domains.

Example environments:

• GitLab Self-Managed: https://gitlab.company.com
• GitHub Enterprise Server: https://github.company.com
• Other Git servers with custom hostnames

A Pro user reported that they could not connect their Enterprise Git repositories, specifically mentioning servers such as gitlab.a.b and github.a.b.

The current experience can make Commit+ appear to support only public SaaS Git hosting.

Goal

Allow users to connect and work with repositories hosted on self-managed Git servers with custom URLs.

Expected behavior

• Users can configure a self-hosted Git provider using its server URL.
• GitHub Enterprise Server and GitLab Self-Managed can be used with custom domains.
• Clone, fetch, pull, push, and other normal Git operations work with these repositories.
• Where provider API integration is supported, repository/account discovery should work against the configured server.
• Authentication should support appropriate credentials/tokens for self-hosted installations.
• Existing GitHub.com / GitLab.com integrations continue to work unchanged.
• Custom hostnames must not be rejected simply because they are not the provider's public SaaS domain.

Scope

At minimum, investigate and support:

• GitHub Enterprise Server
• GitLab Self-Managed

Potentially support other Git-compatible/self-hosted providers later (Gitea, Forgejo, Bitbucket Data Center, etc.).

Acceptance criteria

• Identify all places where provider integrations assume fixed SaaS domains.
• Support configuring a custom GitHub Enterprise Server URL.
• Support configuring a custom GitLab Self-Managed URL.
• Verify authentication against both provider types.
• Verify clone/fetch/pull/push for private repositories.
• Verify repository discovery/API operations where applicable.
• Preserve existing GitHub.com and GitLab.com behavior.
• Add appropriate error handling when a custom server is unreachable or authentication fails.
• Update documentation/help text to clearly state self-hosted Git provider support.

User impact

This is particularly important for enterprise users, where repositories are often hosted on company-controlled GitLab or GitHub Enterprise Server instances rather than public SaaS endpoints.

A Pro user cancelled after finding that their enterprise repositories could not be connected, so this represents a direct product/support gap.

[Tranthanh98]

Resolved in #40