test(e2e): lock the channel content-filter inherit + no-leak (bug 2)

Add feed-content-<key> testids to the toolbar's content pills and two channel
specs: the channel view inherits the feed's Live/Upcoming both ways, and a
content toggle made in the channel view does not leak back into the feed.

frontend/src/components/Feed.tsx

@@ -544,6 +544,7 @@ export default function Feed({
           return (
             <button
               key={c.key}
+              data-testid={`feed-content-${c.key}`}
               onClick={() => setFilters({ ...filters, [c.key]: !on })}
               aria-pressed={on}
               className={`text-xs px-3 py-1.5 rounded-full border transition ${