Main's #6381 deleted the shared .workspace-topbar and scroll-fade rules
from index.css after inlining them at main's own call sites, but this
branch's slim chat chrome still references both classes. The round-8
rebase took the deletion without migrating the branch call sites, so the
header collapsed to zero height — the breadcrumb sat on the window edge,
timeline rows scrolled unfaded through it, and the thread-details popover
anchored to the collapsed header.

Restores both as composable utilities in #6381's own style: a
workspace-topbar utility for the titlebar rows, and the branch's
chat-timeline-scroll-fade mask (soft ramp plus a full-height scrollbar
column). Also drops the duplicated media override and its dead
settings-page-scroll-fade selector.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>