TangerineUI Classic v2.6.4c3

Third Classic release on the Tangerine Neue 2.6.4 base.

Fixes

• "Added to collection" notifications (new in Mastodon 4.6) fully styled — icon, alignment, text layout and colour all had gaps for this new notification type
• Collections tab icon in the notification filter bar now themed correctly (default + active state)

Docs

• README: consolidated and simplified footnotes