Keep a just-read article visible (and Previous-reachable) until the next fetch

The Unread/Older views are live Room Flow queries filtered on
isRead = 0, so marking an article read (which only happens on
Next/Previous, not on open) makes it vanish from the list the instant
Room re-emits -- mid-scroll, or from underneath the reader's own
Previous button, which re-queries this same flow to find its
neighbors. Concretely: Next from C to D marks C read and prunes it
from the live list; Previous from D then can't find C anymore, and
jumps past it to whatever was unread before C. A misclick or the
in-flight prune could cost you the article you meant to keep reading.

Adds ArticleEntity.readAt (set whenever isRead flips true, cleared on
explicit "mark unread"); observeUnread/observeOlderUnread now match
"isRead = 0 OR readAt >= sinceFetch", where sinceFetch is
Settings.lastFetchAt -- the same durable signal the new-articles
snackbar already uses. A read article now lingers (shown dimmed via
the existing showReadState styling) until the next real fetch
actually completes, not the next DB write, and Previous/Next read from
the same still-inclusive query so stepping back to what you just read
finds it right where it was.

0 files changed, 0 insertions, 0 deletions