FEAT: First-class mailing-list view (group by List-Id, summarise threads)

[andrinoff]

Describe the feature

A dedicated "Lists" view that:

• groups inbox by List-Id
• shows per-list unread counts
• collapses thread roots
• adds quick mute / digest-mode actions per list

Why this matters

For users on a dozen high-volume lists, the inbox becomes uninhabitable without per-list summarisation. Mutt has list mode; mailman ships a digest mode; matcha has neither.

Notes

• The existing mailing_lists config can seed this; auto-discover by scanning the last N messages for distinct List-Id values.