When the Agent Panel had the Settings overlay open on top of the
ephemeral new-draft thread, pressing Cmd-N (or clicking Start New
Thread in Settings) silently did nothing — the action ran, but the
Settings overlay stayed painted on top.

The bug was in AgentPanel::activate_draft. Every other activate_*
path goes through set_base_view, which clears the overlay. But
activate_draft has an early return for the case where the base view
already holds the requested draft, and that early return only refocused
— it never touched overlay_view. So when the user was already on the
draft and the Settings overlay was on top, "new thread" had nothing
visible to do.

Fix: in that early-return branch, if an overlay is currently open, clear
it (which also handles focusing and emits ActiveViewChanged).

Added a regression test that puts the panel on the draft, sets the
Settings overlay, dispatches NewThread, and asserts the overlay is
gone. The test directly toggles set_overlay(OverlayView::Configuration, …) rather than calling open_configuration, because the latter builds
provider configuration views and panics on the fake LM provider's
unimplemented!() configuration_view — and the bug being exercised
lives entirely in the overlay/base-view bookkeeping.

Closes AI-237

Release Notes:

• Fixed pressing "New Thread" (or Cmd-N) doing nothing when the Agent Panel's Settings view was open