feat: open the unsynced-changes count to see what it counts

Item 119, and item 146 which is the same request recorded again. The status
bar's count answers "is my work safe to quit on" and could not say what the
work was.

The label opens a read-only list on a click. A QLabel has no clicked signal,
so the press is taken by MainWindow's existing event filter rather than by
replacing the label with a flat QToolButton, which would have brought the
style's button metrics into a status bar the label already sits correctly
in. The pointing-hand cursor is the affordance, since a status-bar label has
room for nothing else.

The layout is the user's own: a message appears once with its actions
beneath it. PendingChangesDialog::rowsFor() does the grouping over a run of
rows sharing an id, which the snapshot has already ordered, so the actions
under one message keep the order they were made in.

Read-only, deliberately. Retrying or discarding a change from here would be
a new mutation path with its own undo question, and the count exists to be
understood rather than edited.

Three rules the tests pin, each of which is a way the list could disagree
with the count it was opened from:

- Grouping must not collapse: two actions on one message are two rows.
- A thread row stays thread-scoped and reports how many messages it covered.
- An id the index no longer holds still opens a run of its own, showing that
  its subject is unknown rather than folding its actions under the message
  above it. This is why the row carries startsMessage rather than inferring
  it from a non-empty subject.

The queued call carrying QStringList, QList<bool> and QList<int> is covered
by a test that drives it across a real thread, since a container whose
metatype does not resolve is dropped at runtime and the slot runs with a
default. Both survive on Qt 6.11; the test is what says so, and what would
fail if that changed.

Italian ships with it: five new strings, lupdate clean, lrelease 522
finished and 0 unfinished.

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

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