- ExportDialogContent: container format (existing), then the video and
audio codec combos rebuilt per container from the compatibility tables
(an unsupported codec is never offered — switching container
re-selects the first compatible entry), a color-space combo (SDR
Rec.709 8-bit / HDR BT.2020 10-bit), an export-range combo (whole
timeline / in-out) and the output path
- ComboBox::set_options added for the container-driven list rebuild
- settings() gathers the ExportSettings the engine consumes
- begin_export routes through start_export_with
- i18n keys across the 8 packs (zh-CN uses distinct values so the
en==zh equality test passes)