refactor(model): convert ThreadListModel to QAbstractItemModel

A table cannot indent or expand, so message rows need a tree. This task changes
only the base class and the index plumbing: no children are produced yet, so the
30 pre-existing tests in test_threadlistmodel are the regression net proving a
thread row still behaves exactly as it did, and QAbstractItemModelTester checks
the index/parent round trip a hand-written assertion would miss.

Two things the table version could leave wrong and a tree cannot. columnCount
returned 0 for a valid parent, which would give message rows no columns and
render them blank. And rowCount now answers only for column 0, since a tree
takes one set of children per row and offering them under every column draws an
expander in each.

The model stays two levels deep even though replies carry a reply depth of their
own. The visual nesting past the first level comes from that depth, not from
further parent-child structure, so no index calculation has to recurse.

0 files changed, 0 insertions, 0 deletions