docs: record items 55 and 58, with their causes corrected

Both backlog entries stated a cause that measurement contradicted, and
both would have misled the next reader into fixing the wrong thing.

Item 55 blamed the thread view's size hint, computed as the sum of its
fixed column widths, and placed the fault on first run. The hint is
256px, not 886: a QTableView does not put its column sum in its size
hint, so that mechanism never applied. The trigger is the restore, in a
window narrower than the one the position was saved in.

Item 58 claimed the zoom range is unenforced and the first render
unusable. MessageView::clampZoom() has always enforced exactly the
README's 0.5 and 3.0; only the report was missing.

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