feat: a list from the cache says how old it is while it reloads

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/config.md

@@ -114,8 +114,9 @@ What lazysecrets has loaded is kept in
 you: the Sources lists, vaults and scopes, and secrets' names and metadata
 (tags, dates, references), never values. On the next start they show at
 once, and anything older than two minutes reloads in the background when
-you go to it, or when Go to searches it. Revealing and copying always
-fetch the value live.
+you go to it, or when Go to searches it. While it does, the pane's
+bottom border says how old what it shows is (`cached 3h ago`). Revealing
+and copying always fetch the value live.
 
 Lists not loaded for 30 days are dropped, and so is everything from an
 account you're no longer signed in to. Repos aren't cached: their files