feat: resolve spell \$d duration to real seconds from SpellDuration.dbc

Spell descriptions now substitute \$d with actual duration values:
  Before: "X damage over X sec"
  After:  "30 damage over 18 sec"

Implementation:
- DurationIndex field (40) added to all expansion Spell.dbc layouts
- SpellDuration.dbc loaded during cache build: maps index → base ms
- cleanSpellDescription substitutes \$d with resolved seconds/minutes
- getSpellDuration() accessor on GameHandler

Combined with \$s1/\$s2/\$s3 from the previous commit, most common
spell description templates are now fully resolved with real values.

Data/expansions/wotlk/dbc_layouts.json

@@ -223,6 +223,7 @@
     "AttributesEx": 5,
     "CastingTimeIndex": 47,
     "DispelType": 2,
+    "DurationIndex": 40,
     "EffectBasePoints0": 80,
     "EffectBasePoints1": 81,
     "EffectBasePoints2": 82,