Add a source picker to search and use the full window width

Search now queries YouTube and SoundCloud together by default and
interleaves the two result lists so neither buries the other; a picker
left of the input narrows it to one source (plus the local library when
configured), and a prefix typed into the query still outranks it.

The panel was capped at 1180px, which left most of a wide screen empty —
it now scales to 1680px and gives the search column the extra room.

A dead link also reported "could not parse yt-dlp's response", which
described our parser rather than the problem; it now shows yt-dlp's own
error line, or says the link did not open.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/api.ts

@@ -42,8 +42,10 @@ export const api = {
 
   state: (serverId: string) => request<ServerStateResponse>(`/api/servers/${serverId}/state`),
 
-  search: (serverId: string, query: string) =>
-    request<{ tracks: Track[] }>(`/api/servers/${serverId}/search?q=${encodeURIComponent(query)}`),
+  search: (serverId: string, query: string, source: string) =>
+    request<{ tracks: Track[] }>(
+      `/api/servers/${serverId}/search?q=${encodeURIComponent(query)}&source=${source}`,
+    ),
 
   play: (
     serverId: string,