feat(rules): a dialog to view and edit the tagging rules

Edits land on a working copy and reach the file only on Save. The
dialog never opens a notmuch database of its own: it publishes the
queries it wants counted and MainWindow runs them through the worker,
because the worker owns the only handle.

Two departures from the drafted version, both of which lost edits.
QPlainTextEdit has no editingFinished, so the note reached the working
copy only for whichever row was current at Save; it is driven from
textChanged instead, with the selection handler blocking the signal so
loading a rule cannot write itself back over the one now current. And
reloadList()'s setCurrentItem emits currentItemChanged, so New and Copy
repopulated the form from m_working before the pending edit had been
flushed into it.

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

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