fix: bullet tax balance + expenses total (v1.4.5)

Three bugs:
1. weeklyExpenses total omitted bulletTax from the sum (both getData paths)
   Fix: add (_exp.bulletTax||0) to the calculation
2. _applyBulletTax read undefined credits as 0 (actors created before the
   credits field existed have null/undefined, not zero)
   Fix: fallback to 500 (template default) only when value is null/undefined
3. Finances tab CURRENT BALANCE showed just C because input had no placeholder
   Fix: add placeholder=500 min=0 to the credits input in character-sheet.hbs

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": "1.4.4",
+  "version": "1.4.5",
   "compatibility": {
     "minimum": 13,
     "verified": "14.359"