feat(filters): make the feed's channel filter reachable

`filters.channelId` was settable by exactly one thing: a "?channel=" URL from a
shared view. The panel still rendered a CHANNEL card whose only control was
"remove" — a remove button for a filter the UI could not apply.

Two ways in now. The panel gets a real Channel group (searchable picker,
first-class alongside date/tags/language, so the chip row surfaces it for free);
normalizeLayout appends it to saved layouts, so nobody loses their arrangement.
And a subscribed channel's page gets "Show in my feed", which applies the filter
and keeps the rest of your filters — the point is "this channel, the way I
normally read", which the channel page itself can't answer (it runs show=all,
scope=all over the whole catalog).

The picker owns its channel query so a few-hundred-entry list loads when the
panel is actually opened, not on every feed visit, and says "showing 50 of N"
rather than quietly stopping.

frontend/src/App.tsx

@@ -843,6 +843,17 @@ export default function App() {
               view={view}
               onBack={closeChannel}
               onOpenChannel={openChannel}
+              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,
+                  channelId: channelView.id,
+                  channelName: channelView.name,
+                });
+                closeChannel();
+                setPage("feed");
+              }}
             />
           </Suspense>
         ) : (