feat(header): floating module header with cyclic nav and shared search

Replace the full-width top bar with a fixed floating header of glass pills: a
fixed-position ModuleName (accent dot + label + prev/next cyclic module stepper)
and a shared SearchBar. The header's left edge is anchored as if the nav rail
and filter sidebar were both open, so it never shifts on collapse or module
change; the right edge leaves a 10% margin.

- ModuleName label width is measured live from the reachable modules' titles in
  the current language, recomputing on a language or account change.
- Module names come from a shared moduleLabelKey (the same short labels as the
  nav rail); NavSidebar and the header now read from one source (lib/modules.ts).
- The arrows step cyclically through moduleOrder(me) — dynamic, no hard-coded
  targets, wrapping at both ends.
- Search is hoisted into the SearchBar for feed (YouTube search), plex, channels
  (including the Discovery tab) and playlists (new); Go button, Enter triggers it.
- The Playlists left rail now reaches the top; header clearance via --hdr-h.

frontend/src/components/ChannelDiscovery.tsx

@@ -17,10 +17,13 @@ import { useConfirm } from "./ConfirmProvider";
 // the channel's metadata — the scheduler pulls its videos on its next run (see backend).
 export default function ChannelDiscovery({
   canWrite,
+  search,
   onOpenWizard,
   onViewChannel,
 }: {
   canWrite: boolean;
+  // Client-side name filter from the shared top SearchBar (matches title + handle).
+  search: string;
   onOpenWizard: () => void;
   onViewChannel: (id: string, name: string) => void;
 }) {
@@ -67,7 +70,10 @@ export default function ChannelDiscovery({
     if (ok) subscribe.mutate(c);
   };
 
-  const rows = query.data ?? [];
+  const q = search.trim().toLowerCase();
+  const rows = (query.data ?? []).filter(
+    (c) => !q || `${c.title ?? ""} ${c.handle ?? ""}`.toLowerCase().includes(q)
+  );
 
   const columns: Column<DiscoveredChannel>[] = [
     {