feat: add and standardize OAuth provider support

[appleboy]

• Introduce full GitLab OAuth support, including provider implementation, configuration, bootstrap wiring, and login UI icon
• Refactor Gitea user info fetching to use a precomputed API URL instead of deriving it from the auth endpoint
• Add a shared apiURL field to OAuth providers for cleaner and more consistent user info requests
• Add comprehensive test coverage for GitHub, Gitea, GitLab, and Microsoft OAuth providers, including success paths and error cases
• Improve provider constructors to normalize base URLs by stripping trailing slashes
• Extend provider dispatch logic to handle GitLab and add tests for unsupported providers and display names

[codecov]

Codecov Report

❌ Patch coverage is 72.13115% with 17 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/bootstrap/oauth.go	10.52%	17 Missing ⚠️

📢 Thoughts on this report? Let us know!

[Copilot]

Pull request overview

This PR adds GitLab as a first-class OAuth provider and standardizes instance-based OAuth providers by introducing a precomputed apiURL for user-info fetching, alongside expanded provider test coverage.

Changes:

• Add GitLab OAuth support end-to-end (config/env, bootstrap wiring, provider implementation, login-page icon).
• Refactor Gitea user-info fetching to use a precomputed API URL and add apiURL to provider struct for consistency.
• Add/expand unit tests across OAuth providers (GitHub, Gitea, GitLab, Microsoft), including success and error paths.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/templates/login_page.templ	Adds GitLab icon rendering for OAuth login buttons.
internal/config/config.go	Adds GitLab OAuth configuration fields and env loading.
internal/bootstrap/oauth.go	Wires GitLab provider initialization into bootstrap/provider registry.
internal/auth/oauth_provider.go	Adds apiURL, normalizes base URLs, adds GitLab provider constructor, dispatches GitLab user info.
internal/auth/oauth_gitea.go	Switches Gitea user-info fetch to use p.apiURL.
internal/auth/oauth_gitlab.go	Implements GitLab /api/v4/user user-info fetch and mapping.
internal/auth/oauth_provider_test.go	Adds shared testing utilities and tests for display name + unsupported providers.
internal/auth/oauth_github_test.go	Adds GitHub provider constructor + user-info tests (including email fallback behavior).
internal/auth/oauth_gitea_test.go	Adds Gitea provider constructor + user-info tests (including error cases).
internal/auth/oauth_gitlab_test.go	Adds GitLab provider constructor + user-info tests (including error cases).
internal/auth/oauth_microsoft_test.go	Adds Microsoft provider constructor + user-info tests (including error cases).

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