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/lib/panelLayout.ts

@@ -16,7 +16,7 @@ export interface PanelLayout {
 // Unknown/stale ids are dropped and any missing (e.g. a group added in a later version) are
 // appended by normalizeLayout, so nothing silently disappears.
 const PANEL_ORDER: Record<PanelId, string[]> = {
-  feed: ["savedviews", "date", "tags", "language", "topic"],
+  feed: ["savedviews", "date", "channel", "tags", "language", "topic"],
   plex: [
     "scope",
     "playlists",