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