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.

pysim_simple_server/server.py

@@ -32,7 +32,7 @@ from osmocom.tlv import BER_TLV_IE
 from osmocom.utils import rpad
 
 
-VERSION = '3.13.3'
+VERSION = '3.13.4'
 
 MAX_ENVELOPE_SEGMENTS = 5  # max SMS segments for outgoing C-APDU in ENVELOPE