fix(channel): "Show in my feed" adds to the picked channels, not replaces

Sprint 5 made the channel filter multi-select, but this handler still wrote a
single-element array — so arriving from a second channel page silently dropped
the first. Union the id in (deduped, so repeating it for an already-picked
channel is a no-op).

frontend/src/App.tsx

@@ -847,7 +847,13 @@ export default function App() {
               onShowInFeed={() => {
                 // Keep the rest of the feed's filters — the point is "this channel, the way I
                 // normally read": unwatched, my tags, my date window.
-                setFilters({ ...filters, channelIds: [channelView.id] });
+                // ADD to the picked set, don't replace it: channels OR together now, so arriving
+                // from a second channel page should widen the feed, not silently drop the first.
+                // Deduped — clicking it again for a channel already picked is a no-op.
+                setFilters({
+                  ...filters,
+                  channelIds: [...new Set([...filters.channelIds, channelView.id])],
+                });
                 // setPage already leaves an open channel page and pushes a clean history entry.
                 // Calling closeChannel() first ran history.back(), whose popstate landed AFTER this
                 // and restored whatever page the channel had been opened from.