Skills: +-5 buttons, advanced skills use 50% of the attribute (creation.advanced_attr_percent)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HJhXuLD2Z6hv3Wqeq9UL9K

backend/data/rules.json

@@ -15,6 +15,7 @@
     "skill_points_default": 150,
     "skill_max_divisor": 2,
     "advanced_unlock_cost": 10,
+    "advanced_attr_percent": 50,
     "free_proficiencies": 2,
     "max_proficiency_pips": 2,
     "talents_min": 1,
@@ -41,6 +42,7 @@
     {"key": "profession_attr_bonus", "label": "Profession attribute bonus", "group": "Profession", "min": 0, "max": 50},
     {"key": "free_proficiencies", "label": "Free proficiency picks", "group": "Skills", "min": 0, "max": 20},
     {"key": "skill_points_default", "label": "Skill point pool", "group": "Skills", "min": 0, "max": 1000},
+    {"key": "advanced_attr_percent", "label": "Advanced skills: share of the attribute that counts (%)", "group": "Skills", "min": 0, "max": 100},
     {"key": "advanced_unlock_cost", "label": "Cost to unlock an advanced skill", "group": "Skills", "min": 0, "max": 100},
     {"key": "skill_max_divisor", "label": "Skill maximum = attribute divided by", "group": "Skills", "min": 1, "max": 10},
     {"key": "proficiency_bonus", "label": "Proficiency bonus", "group": "Skills", "min": 0, "max": 50},
@@ -434,7 +436,7 @@
           "placeholder": true
         },
         {
-          "p": "The unlock cost for advanced skills (10 points) is a house rule of the app, not part of the brainstorm. It can be changed in rules.json (creation.advanced_unlock_cost).",
+          "p": "The unlock cost for advanced skills (10 points) and the rule that advanced skills only use half of the attribute are house rules of the app, not part of the brainstorm. They can be changed in rules.json (creation.advanced_unlock_cost, creation.advanced_attr_percent).",
           "placeholder": true
         },
         {