feat(completion): render the popup with descriptions

The free-form date hint is a footer label rather than a model row: a row
would be filtered away by the first non-matching keystroke and could be
selected and inserted, producing a query that errors.

QCompleter::setPopup takes a QAbstractItemView, so the label cannot be laid
out beside the view in a container widget. The footer sits in space reserved
with setViewportMargins inside the list view instead.

setItemDelegate must run after setPopup, not before: setPopup installs a
plain QStyledItemDelegate of its own and discards whatever was already set,
which silently drops the description column.

Accepting replaces exactly the span the tokenizer identified rather than
QCompleter's own completion prefix, which is a different span once a prefix
or a range bound is involved. Four tests drive that path directly instead of
through synthetic key events, since whether a key needs Shift is a
keyboard-layout property and could not decide the question.

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

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