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/i18n/locales/en/channel.json

@@ -23,5 +23,6 @@
   "country": "Country",
   "language": "Language",
   "topics": "Topics",
-  "keywords": "Keywords"
+  "keywords": "Keywords",
+  "showInFeed": "Show in my feed"
 }

frontend/src/i18n/locales/en/sidebar.json

@@ -40,7 +40,8 @@
     "content": "Content type",
     "language": "Language",
     "topic": "Topic",
-    "tags": "Your tags"
+    "tags": "Your tags",
+    "channel": "Channel"
   },
   "show": {
     "unwatched": "Unwatched",
@@ -72,5 +73,8 @@
     "1month": "1 month",
     "6months": "6 months",
     "1year": "1 year"
-  }
+  },
+  "channelSearch": "Search channels…",
+  "noChannelMatch": "No channel matches.",
+  "channelMore": "Showing {{shown}} of {{total}} — search to narrow."
 }

frontend/src/i18n/locales/hu/channel.json

@@ -23,5 +23,6 @@
   "country": "Ország",
   "language": "Nyelv",
   "topics": "Témák",
-  "keywords": "Kulcsszavak"
+  "keywords": "Kulcsszavak",
+  "showInFeed": "Mutasd a hírfolyamomban"
 }

frontend/src/i18n/locales/hu/sidebar.json

@@ -40,7 +40,8 @@
     "content": "Tartalomtípus",
     "language": "Nyelv",
     "topic": "Téma",
-    "tags": "Saját címkék"
+    "tags": "Saját címkék",
+    "channel": "Csatorna"
   },
   "show": {
     "unwatched": "Nem nézett",
@@ -72,5 +73,8 @@
     "1month": "1 hónap",
     "6months": "6 hónap",
     "1year": "1 év"
-  }
+  },
+  "channelSearch": "Csatornák keresése…",
+  "noChannelMatch": "Nincs találat.",
+  "channelMore": "{{total}} közül {{shown}} látszik — keress a szűkítéshez."
 }