fix: prevent duplicate credits submission causing NaN corruption

The non-Mk2 sheet had two inputs named system.details.credits (header
+ finances tab). Foundry concatenates duplicate names as "500,500"
which is not a valid number, causing the field to show blank/grey
after saving.

Fix: finances tab input made readonly with no name attribute — header
is now the sole authoritative credits field.

Also: on-boot sanitization now covers credits, fixing any actors
already storing the corrupted comma-string value.

system.json

@@ -2,7 +2,7 @@
   "id": "zero-engine",
   "title": "Zero Engine (SLA Industries)",
   "description": "A basic game system for SLA Industries using the Zero Engine rules.",
-  "version": "1.4.9",
+  "version": "1.5.0",
   "compatibility": {
     "minimum": 13,
     "verified": "14.359"