fix(app): use timeline accent for unread divider

[brsbl]

Summary

• add a semantic timeline-accent theme token for the timeline
• map it to bb’s existing soft paper-blue timeline tint in the default theme and to each built-in palette’s blue/cyan accent
• keep file-accent aliased to timeline-accent so existing file-path styling stays aligned
• use timeline-accent for timeline title accents and the unread divider instead of destructive red

Rationale

• Red/destructive remains reserved for errors and destructive states.
• primary is not the right semantic fit here: in the default bb theme it is neutral, and in custom palettes it is the general product/control accent.
• The timeline already has one chromatic blue/cyan affordance for link-like timeline targets, so timeline-accent makes the unread marker match that visual language without borrowing an error token or a file-specific name.

Validation

• pnpm exec turbo run test --filter=@bb/app -- src/components/ui/theme.test.ts
• pnpm exec turbo run typecheck --filter=@bb/app