fix(app): dismiss session sidebar on selection for narrow displays

[bmpenuelas]

Issue for this PR

Fixes #37746
References #24577
Supersedes #14709

Type of change

• Bug fix
• [ ] New feature
• [ ] Refactor / code improvement
• [ ] Documentation

What does this PR do?

On mobile, selecting a session from the session sidebar can leave the sidebar fully covering the chat view. This makes it impossible for the user to click/tap the session to dismiss the sidebar, so they feel locked.

This keeps the existing behavior on wider layouts, where the user can still tap/click outside the sidebar. On narrow layouts where the sidebar is effectively full-width, selecting a session now dismisses it so the selected session becomes visible.

The fix reuses the existing mobile sidebar dismissal path and adds a regression test for narrow session selection.

How did you verify your code works?

• bun: run app typecheck and tests for the parts affected by this PR. There are preexisting errors outside the scope of this PR which have been left untouched.
• Manual: narrow viewport, open the session sidebar, select a session, confirm the sidebar closes and the selected session is visible.
• Manual: wider viewport, select a session and confirm the current sidebar behavior is preserved.

Screenshots / recordings

No UI changes, only UI-behavior changed.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR