Setting for where New project creates folders ("New projects are created in")

[areidyOTH]

Problem

New project always creates its folder at <T3 home>/projects/<slug> (normally ~/.t3/projects/<slug>). No setting changes this. If you keep your repositories somewhere else, like a data drive, you end up with projects in two places. The only ways to change the location today are:

• symlinking ~/.t3/projects, or
• moving the whole T3 home with T3CODE_HOME / --base-dir, which also moves the database.

In #14960 the maintainers agreed that this shouldn't reuse "Add project starts in", since that setting only controls where the folder browser opens. They suggested a separate setting instead. This is that proposal.

Proposal

Add a server-scoped setting, New projects are created in, next to "Add project starts in" in Settings.

• Empty (default): keep today's behaviour, <baseDir>/projects. Nothing changes for existing users.
• Set: New project creates <configured dir>/<slug>.
• Scope: per environment/server, like the root itself. Each machine has its own filesystem, so a single global value wouldn't make sense (see [Bug]: "Add project starts in" is a per-environment path exposed as a global setting, so local and remote environments cannot differ #11756).

How it fits what's already there

• The server already sends ServerConfig.newProjectsRoot to clients. The web, desktop and mobile apps already use it for the Goes in <root> / Creates <path> preview. If the server computes newProjectsRoot from the setting (falling back to <baseDir>/projects), the clients mostly just need the settings row.
• namedProjectsRoot in ManagedProjectFolders would read the setting instead of hard-coding path.resolve(config.baseDir, "projects").
• Existing projects aren't affected because their paths are stored. Changing the setting only applies to projects created after the change.

Edge cases to decide

• Validation: require an absolute path. If the folder is missing, either create it on first use or show an error in Settings.
• Not writable or unmounted (e.g. a removable drive): New project should fail with a clear error, not fall back to ~/.t3/projects without telling you.
• Root inside an existing git repo: this should be fine, because each new project runs its own git init. That's the same reasoning the code comment already gives for ~/.t3/projects.
• Same folder as "Add project starts in": allowed. The settings stay independent; a user can simply give both the same value.

Alternatives considered

• Reuse addProjectBaseDirectory: rejected in Create New Project ignores "Add project starts in" and always uses ~/.t3/projects #14960 because it mixes up "where the browser opens" with "where T3 creates repos".
• Symlink ~/.t3/projects: works, but it's invisible in the UI and easy to break.
• Move the T3 home: this moves the database and other data too, which is far more than needed.

Related: #14960, #14527, #14569, #11756

[jorgitin02]

I would also like an option to change the destination folder directly in the New project dialog, not only a setting for the default root.

Suggested flow:

• Enter the project name.
• Show the destination path, initially using the configured default (or the current <T3 home>/projects fallback).
• Offer Change folder / Browse to select a different parent folder for this project, with manual path entry as an alternative.
• Create the new project under the selected parent folder.

For a desktop client connected to a remote server, the folder picker should browse the selected server’s filesystem, rather than the client machine’s filesystem. For example, using T3 on macOS with a Linux homeserver should let me choose where the repository is created on that homeserver.

Keep the existing location as the default, and leave existing projects untouched. A configurable per-server default plus a per-project override would cover both workflows without needing to create a folder separately and then use Add project.