v3.13.4: Cards editor counter field: explicit heading, placeholder, validation

The keyset table's column is headed "Counter (hex)" with the placeholder
0000000001, and cardsAdd refuses a save whose counter is not 1-10 hex
digits (empty included; whitespace stripped) - the same rule the store
enforces.  The store keeps its import-compatible empty -> 1 default for
API/import callers.

Docs: repo AGENTS (v3.13.4 note).  sw cache simple-v374.
Tests: 667 Python + 797 frontend green.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "3.13.3"
+version = "3.13.4"
 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.