[Feature]: Switch the sidebar project filter when starting a thread in another project

[park-robins]

Area

Thread sidebar / project filtering

Problem or use case

When the left sidebar's thread list is filtered to Project A and I start a new thread in Project B, the sidebar stays filtered to Project A. I am now working in Project B, but the thread list still shows the previous project's threads, so I have to switch the filter manually.

Proposed solution

When I start a new thread in a different project, automatically switch the sidebar's project filter to the project I selected for that new thread.

Example:

1. Filter the left sidebar to Project A.
2. Start a new thread and choose Project B.
3. The sidebar filter switches to Project B and shows Project B's threads, including the new thread once it exists in the list.

The change should happen when the new-thread project is selected, so the sidebar matches the project I am starting work in.

Why this matters

Starting a thread in another project is an explicit change of working context. Having the filtered thread list follow that choice removes an extra manual step and keeps the current thread visible in its project's list.

Related requests

#11895 and Discussion #14185 ask for new threads to inherit the sidebar's current project. This request covers the reverse direction: when I explicitly choose a different project for a new thread, update the sidebar filter to that project.