ui: stop showing the all-FF record marker as a field; label it in the diff (v3.1.4)

A profile check comparing an empty PNN record against a populated one
showed a cryptic 'Empty: yes' row: the decoders return {empty: true} for
an all-FF record and efFlatten() rendered that marker as a pseudo-field
('empty' prettified to 'Empty', the boolean as 'yes').

- efFlatten() skips the marker (the decoded views already show their own
  translated 'empty' note next to the record).
- efDiffData() fills the cells of a side that decodes to an all-FF record
  with 'Empty (all FF)' (RU: 'Пусто (только FF)') instead of leaving them
  blank; a field merely missing from a non-empty record stays blank.
- tests: efFlatten marker, one-sided and both-sided empty diffs, and the
  PNN case from the report; help EN/RU note the new text.
- sw.js simple-v239.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "3.1.3"
+version = "3.1.4"
 description = "HTTP REST server wrapping pysim for the SIMple PWA"
 requires-python = ">=3.8"
 # pysim is a git-only dependency installed explicitly by setup.bat/setup.sh.