Mobile: allow removing a project from the project picker

[bawisniewski]

Problem

Mobile currently has no durable way to remove a project after it has been added. The only removal path is the temporary clone banner on the new-task screen, so once that disappears, a project added by mistake remains in the picker unless the user switches to web or desktop.

This is particularly easy to hit when a broad directory, such as a home folder, is selected accidentally.

Proposed scope

Add one removal entry point to the existing Choose project sheet:

• iOS: swipe a project row left and choose Remove.
• Android: long-press a project row and choose Remove project.
• Show a confirmation containing the project path, environment, and affected thread count.
• Remove the project and its threads using the existing project.delete contract.

The project picker is the narrowest useful surface because it is already where mobile lists all projects.

Non-goals

• Project rename.
• Additional project-management controls in Settings.
• Contract or server changes.

A draft implementation is available in PR #12579. I am narrowing that PR to this removal-only scope before requesting review.

Would this fit the projects maintainers want mobile to support?