fix(web): proxy Apple charts (CORS) + clear message when Apple search is blocked

Confirmed via curl from a clean IP: Apple's iTunes /search returns 403 (blocked
server-side, likely datacenter-IP anti-scraping) while /lookup?id= still returns 200.
So the Erkunden Apple search fails through no fault of ours.

- Charts: the "most-played" RSS (rss.applemarketingtools.com) was fetched directly
  from the browser and hit CORS; proxy it via /api/itunes-charts/:kind.
- Error visibility: itunesSearch records the last outcome (HTTP status / network);
  the global-search view now shows "Apple-Suche gerade nicht verfügbar" with the
  reason (403 = Apple blocks the API, not you) instead of a blank "no results".
- iTunes proxy now forwards the upstream status code instead of always 200.

Deezer (free, no key, ~50 req/5s) is the planned search fallback; MusicBrainz is too
rate-limited (1/s) for interactive search.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v47';
+const CACHE = 'ns-v48';
 const PRECACHE = ['/', 'index.html', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];
 // Auth token params stripped from cache keys so artwork survives across sessions/token refreshes.