fix: KiwiSDR layer broken import + remove ugly iframe embed

- kiwisdr_fetcher.py imported non-existent `smart_request` (renamed to
  `fetch_with_curl`), causing silent ImportError → 0 nodes returned
- Replaced KiwiSDR iframe embed with clean "OPEN SDR RECEIVER" button.
  The full KiwiSDR web UI (waterfall, frequency controls, callsign
  prompt) is unusable at 288px — better opened in a new tab.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Former-commit-id: aa0fcd92b2390d6a8943b68f2f7eb9b900c7bbb7

backend/services/kiwisdr_fetcher.py

@@ -34,10 +34,10 @@ def _parse_gps(html: str):
 @cached(kiwisdr_cache)
 def fetch_kiwisdr_nodes() -> list[dict]:
     """Fetch and parse the KiwiSDR public receiver list."""
-    from services.network_utils import smart_request
+    from services.network_utils import fetch_with_curl
 
     try:
-        res = smart_request("http://kiwisdr.com/.public/", timeout=20)
+        res = fetch_with_curl("http://kiwisdr.com/.public/", timeout=20)
         if not res or res.status_code != 200:
             logger.error(f"KiwiSDR fetch failed: HTTP {res.status_code if res else 'no response'}")
             return []