New de/en/es keys for #827: the Datum·Ereignis·Thema segment labels and their
≤320px abbreviations, the dynamic meta-line grouping labels
(timeline_grouping_event/_thema), the "Weitere Briefe"/"Ohne Thema" bucket
labels, the radiogroup aria-label, the letters-hidden disabled reason, and the
multi-tag hint. Reuses the existing timeline_grouping_date / timeline_tag_chip_label.
Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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