feat(model): resolve action scope from the selected row kind

ActionScope is what an action is about to touch, resolved from the selection in
one place so no call site reinvents the mapping. A thread root contributes the
whole thread, a message row contributes one message, and messageCount is what
the status bar reports.

The count comes from totalCount, not from the loaded children. A thread that was
never expanded still has all of its messages, and counting only the rows that
happen to be on screen would understate what the action does: mutation-checked,
and the wrong version reports 1 message where 7 are about to be tagged.

A mixed selection is honoured as given rather than escalated to thread scope or
narrowed to message scope. Silently widening it would defeat the reason the
scope is shown at all.

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