fix: keep bulk tag actions linear, and say when they land

Select all on a 10k-row search, then Archive, froze the window with one
CPU pinned. gdb on the live process showed the UI thread cubic before the
write was ever sent: each optimistic row write emitted dataChanged, whose
handler refreshed the action labels by walking the whole selection, and
each step of that walk looked the message up with a linear scan of every
thread.

ThreadListModel now answers id lookups from a lazily rebuilt hash,
invalidated by its own structural signals and by hand at the two in-place
id changes. MainWindow::batchModelWrites() holds the label refresh while a
loop of optimistic writes runs, then refreshes once. Two list membership
tests became sets.

The announcement also used to fade on a timer whether or not the write
had landed. It now reads "applying" until a round trip through the
worker's queue says the write has run, then "done". An error takes the
label back from a late "done".

Item 211.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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