feat(ui): make Escape deselect the row as well as blank the pane

Blanking the pane while leaving the row highlighted reads as half an
action, and deselecting is what Escape means nearly everywhere else. The
user asked for two actions rather than a changed one, so clear_pane keeps
its behaviour and moves to Shift+Esc; clear_selection takes Escape and
does both. Shift+Esc rather than unbound because every action carries a
default and a test enforces it.

Clearing the selection re-adopts the thread it just cleared, unless done
in exactly the right way. clearSelection() leaves currentIndex() valid,
so onSelectionChanged takes its "one or fewer rows" branch, sees a
current row whose id differs from m_currentThreadId, and calls
onThreadSelected for it. Clearing the selection before blanking lets that
run while the id still matches, so nothing reloads, and clearing current
stops a later collapse-to-one-row reaching the same row. All four
arrangements were tried; only this one passes.

The first version of the test could not distinguish any of them. It
asserted showingPlaceholder(), which passes regardless because this
fixture has no worker, so loadThread never replies and the pane is never
repainted. currentThreadId() and currentIndex() are observable without
one, and asserting those is what made the test discriminate.

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