ui: drop the network-monitor area line, label the refresh time (v2.7.5)

The second line under the location (e.g. "RAI FFFE") showed only the LAC
part of the current area identity: after every dummy-writing scenario it
was the constant FFFE, and otherwise it was redundant with the
EF.LOCI/PSLOCI/EPSLOCI rows below.  netStateRender no longer renders it.

The timestamp line now carries a translated "Last refresh" label
('Last refresh': 'Последнее обновление').

- frontend/index.html: remove #netstate-area (markup + render), label the
  read time, LANG_RU entry
- frontend/tests: html test pins the label and the absence of the area
  line; netstate test asserts the rendered timestamp
- sw.js cache simple-v208; version trio 2.7.5

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "2.7.4"
+version = "2.7.5"
 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.