STATUS list entries: suppress raw hex when empty, describe FCP data

- decode.py: _build_summary emits 'Data: N bytes — FCP of <file> (...)' for
  STATUS responses with data (FCP already decoded into decoded.response);
  _fcp_brief builds the 'FCP of MF (DF or ADF)' / 'FCP of EF_xxx (...)'
  fragment
- index.html: cmd_description suppresses the raw-hex fallback for
  body-less STATUS rows (nothing to describe beyond type + SW)
- tests: no-data STATUS summary None; DF/EF FCP summaries; pluralization
- Bump 1.22.19, SW cache v50->v51

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simtrace-analyser-v50';
+const CACHE = 'simtrace-analyser-v51';
 
 const PRECACHE = [
     'index.html',