docs: reconcile the backlog, and specify card avatars

Reconciliation against the user's notes found two unrecorded lines, and
corrected the cause of one entry that was recorded wrongly.

- 169, new: a card shows the account only as a bar, with no fade and no
  avatar. Half of it shipped as the accent bar.
- 170, new: a row that stops matching the view only leaves it on the
  Delete path. Filed from a note that reads as a stale request for
  optimistic updates; it is not. removeThreadsWithoutTag() has exactly
  one caller, so marking a message read in the Unread view repaints the
  row and leaves it in a list it no longer belongs to.
- 65, narrowed: the notes now name it as a dead-code and duplication
  sweep rather than a performance or security pass, so it produces a
  list to decide on rather than a diff.

The spec for 169 covers the avatar geometry, the fade, the two
hash-generated fills and the sender list that chooses between them. It
also records the one structural change the feature needs and the
measurement that forced it: ThreadSummary::authors carries display names
only, with no address anywhere, so the identicon has nothing stable to
hash and the sender list has nothing to match. ThreadSummary gains
firstMessageSender, filled by the walk that already fills firstMessageId.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LXCZFLXbAii5n5wtovpdhh

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