The grouped view flooded: buckets had no visual containment (a tiny floating pill
over cards identical to the ungrouped view) and the >12-letter density collapse was
gone, so "Weitere Briefe · 325" / "Sonstiges · 10" dumped every card.
LetterBucket now binds each cluster with a coloured left rail (tag colour in Thema,
mint for an Ereignis cluster, neutral for the fallback), renders compact cards, and
— above BUCKET_DENSE_THRESHOLD (6) — collapses to the existing month-density
YearLetterStrip instead of a flood. Adds a `nested` mode (no header) for letters that
sit under their event pill, and shares the tag-colour token allow-list via tagColorVar.
Refs #827
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