profiler editor: keep record/content fields visible

- Seed one empty record row when record content is created or reset
  (filetype change or Contents None->Exact/Mask) via profilerEmptyRecordContent
- Add per-record delete and '+ Record' add controls so the record view is
  always editable; delete is hidden on the last remaining row

SW cache v47.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v46';
+const CACHE = 'otaman-v47';
 const URLS = [
   'index.html',
   'help.html',