feat: snapshot the pending changes as rows

Item 119, first half: the data the list behind the unsynced-changes count
is built from, with no dialog and no worker, so the rules it has to follow
are testable on their own.

pendingChangeSnapshot() gathers the three queues the count sums into
PendingChange rows. Two properties are the whole point.

Scope follows the ACTION, not the storage. A held thread edit stays one
thread row, because a `*_thread` action made it and reporting its messages
instead would claim the user acted on each one; a netted tag edit and a held
move are message rows. The queues already encode that distinction, so
nothing is expanded and nothing is escalated.

The rows are grouped by id, so a message with several outstanding actions
appears once with its actions beneath it, which is the layout the user
asked for. The sort is stable, so those actions keep the order they were
made in; QHash has none of its own, and without it the list would reshuffle
between openings.

A snapshot, taken once and frozen. Subjects are empty here and filled by the
resolve step to come.

m_pendingTagEdits gains the action name beside the direction it already
kept. The direction alone was enough to count with; a list has to say what
each change was, and only the action that made it knows. It is carried from
TagChange::description rather than derived from the tag, so there is no
second table of tag names to labels to drift from the first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P88Q3MCSCSQxKDy7pmXh9F

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