profiler: compact ignore-contents list in scan dialog

The scan dialog now shows a single 'Ignore contents of files:' label and a
two-column checkbox grid with just the file names (the per-row 'Ignore
contents of ...' prefix was redundant). Drop the now-unused i18n key and
add the new label (RU: 'Игнорировать содержимое файлов:'). Help docs
updated. SW cache v67 -> v68.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v67';
+const CACHE = 'otaman-v68';
 const URLS = [
   'index.html',
   'help.html',