feat(server): surface and configure a project’s default branch

Not tied to an existing issue.

What & why

A project’s default branch is the baseline for a surprising amount of logic: bundle size comparisons, flaky-test detection / quarantine eligibility, test sharding timing data, and latest-preview resolution all key off it. But there was no way to view or change it from the dashboard — it defaulted to main and was silently wrong for any project whose real default branch is something else (master, develop, trunk, …), quietly pointing all of those baselines at a branch with no data.

This PR:

• Surfaces the default branch in project General settings as a card with the current branch shown as a badge.
• Lets users edit it via an “Edit default branch” modal (pencil icon trigger, matching the test automations edit affordance).
• Auto-syncs from GitHub on connect: when a project is connected to a GitHub repository, Tuist adopts the repository’s default_branch. The repository list is already loaded with default_branch, so this is a free metadata copy with no extra API call. Only the initial connection syncs, so a later manual edit is preserved.

The settings field is the manual fallback (self-managed repos, projects not connected to GitHub); the GitHub sync covers the common case with zero user effort.

Root cause / gotcha worth knowing

Blank input is rejected in the LiveView handler before it reaches the changeset. Ecto.Changeset.cast substitutes the schema default (main) for an empty string, so a naive “clear the field and save” would have silently reset the branch to main rather than erroring — and validate_required could never catch it. The handler guards the blank case and surfaces a “can’t be blank” error instead. update_changeset also caps the branch length.

User / developer impact

• Users can finally see and set the baseline branch their analytics, insights, and comparisons run against.
• GitHub-connected projects get the correct branch automatically.
• No schema/migration change — default_branch already existed; this just exposes and populates it.

How to test locally

1. Open a project’s Settings → General. You’ll see a Default branch card with the current branch as a badge and a pencil edit button.
2. Click the pencil, change the branch in the modal, Save — the badge updates and persists across reloads.
3. Clear the field and save — it’s rejected with “can’t be blank” and the value is preserved.
4. Connect a GitHub repository (Integrations) to a project — the project’s default branch is set from the repository’s default branch.

Validation run: mix test test/tuist_web/live/project_settings_live_test.exs test/tuist_web/live/integrations_live_test.exs test/tuist/projects_test.exs (105 tests, 0 failures), mix credo, mise run format, and a manual pass through the full UI flow (edit, persist, blank-reject, GitHub sync).

Changelog

Includes a marketing changelog entry (“Configurable default branch”). The pull_request field is intentionally omitted for now — I’ll add this PR number before merge.

🤖 Generated with Claude Code