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/components/ChannelPage.tsx

@@ -1,7 +1,7 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
-import { ArrowLeft, Ban, Check, ExternalLink, Loader2, Plus, RefreshCw } from "lucide-react";
+import { ArrowLeft, Ban, Check, ExternalLink, Loader2, Plus, RefreshCw, SlidersHorizontal } from "lucide-react";
 import Avatar from "./Avatar";
 import Feed from "./Feed";
 import { useConfirm } from "./ConfirmProvider";
@@ -56,12 +56,17 @@ export default function ChannelPage({
   view,
   onBack,
   onOpenChannel,
+  onShowInFeed,
 }: {
   channelId: string;
   initialName?: string;
   me: Me;
   view: "grid" | "list";
   onBack: () => void;
+  // Narrow the real feed to this channel and go there. Offered only for a channel you're
+  // subscribed to: the feed defaults to scope "my", so filtering it to a channel you don't
+  // follow would just land you on an empty page.
+  onShowInFeed: () => void;
   onOpenChannel: (id: string, name?: string) => void;
 }) {
   const { t, i18n } = useTranslation();
@@ -290,6 +295,16 @@ export default function ChannelPage({
             >
               <ExternalLink className="w-4 h-4" />
             </a>
+            {ch?.subscribed && (
+              <button
+                onClick={onShowInFeed}
+                title={t("channel.showInFeed")}
+                aria-label={t("channel.showInFeed")}
+                className="inline-flex items-center justify-center w-9 h-9 rounded-lg text-muted hover:text-accent border border-border hover:border-accent transition"
+              >
+                <SlidersHorizontal className="w-4 h-4" />
+              </button>
+            )}
             {!me.is_demo && (
               <button
                 onClick={() => block.mutate()}