feat(tags): mark every thread in the view read, in one undoable step

An action removing "unread" from every thread in the current view,
on the toolbar, the Message menu and Ctrl+Shift+U. It deliberately
ignores the selection, which makes it the one action in the window
that does, and it routes through the same funnel as every other tag
change, so it is one write rather than one per thread.

Disabled until the query reports its total. Threads arrive in batches,
so before then the model holds only what has landed, and an action
saying "all" must not silently skip the rest. A greyed control says
"not yet" without needing a dialog or a stall the user cannot see.
The state is also set at registration, since QAction starts enabled
and a window that has not run a query has nothing to act on.

Two things came out differently from the plan, both forced by existing
code. It carries a default binding, because everyActionHasAShortcut
requires every registered action to have one: an unbound action is
unreachable from the keyboard, and that invariant is deliberate, so the
action was given Ctrl+Shift+U rather than the invariant relaxed. And
only the threads that are actually unread are sent, because sending the
rest would inflate the pending-edit count with writes that change
nothing, and the quit prompt reads that count. A view with nothing
unread does nothing, pushes no command and says so: an undo entry that
restores nothing is worse than none, since it absorbs a Ctrl+Z meant
for the previous action.

undoDepthForTesting() is new and exists for a reason worth recording:
undo->isEnabled() cannot answer "was a command pushed", because the
undo QAction is always enabled and tests canUndo() when triggered. The
first version of the no-op test asserted on it and passed against a
mutant with the unread filter removed.

Closes item 43.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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