refactor: extract components from SkillsTab.tsx to reduce below 400 lines

src/lib/game/hooks/useSkillUpgradeSelection.ts

@@ -3,6 +3,7 @@
 
 import { useState, useMemo, useCallback, SetStateAction, Dispatch } from 'react';
 import type { SkillUpgradeChoice } from '@/lib/game/types';
+import { getUpgradesForSkillAtMilestone } from '@/lib/game/skill-evolution';
 
 export interface UseSkillUpgradeSelectionResult {
   pendingSelections: string[];
@@ -12,6 +13,36 @@ export interface UseSkillUpgradeSelectionResult {
   handleCancel: (onClose: () => void) => void;
 }
 
+/**
+ * Check if skill has milestone available (also exported for use in SkillRow/CategorySkillsList)
+ */
+export function hasMilestoneUpgrade(
+  skillId: string,
+  level: number,
+  skillTiers: Record<string, number>,
+  skillUpgrades: Record<string, string[]>
+): { milestone: 5 | 10; hasUpgrades: boolean; selectedCount: number } | null {
+  // Check level 5 milestone
+  if (level >= 5) {
+    const upgrades5 = getUpgradesForSkillAtMilestone(skillId, 5, skillTiers);
+    const selected5 = (skillUpgrades[skillId] || []).filter((id) => id.includes('_l5'));
+    if (upgrades5.length > 0 && selected5.length < 2) {
+      return { milestone: 5, hasUpgrades: true, selectedCount: selected5.length };
+    }
+  }
+
+  // Check level 10 milestone
+  if (level >= 10) {
+    const upgrades10 = getUpgradesForSkillAtMilestone(skillId, 10, skillTiers);
+    const selected10 = (skillUpgrades[skillId] || []).filter((id) => id.includes('_l10'));
+    if (upgrades10.length > 0 && selected10.length < 2) {
+      return { milestone: 10, hasUpgrades: true, selectedCount: selected10.length };
+    }
+  }
+
+  return null;
+}
+
 /**
  * Hook for managing skill upgrade selection state in the SkillsTab milestone upgrade dialog.
  * Manages pending selections across the dialog open/close cycle.

src/lib/game/types/index.ts

@@ -20,7 +20,8 @@ export type {
   SkillTierDef, 
   SkillPerkChoice, 
   SkillUpgradeChoice,
-  PrestigeDef 
+  PrestigeDef,
+  SkillCost
 } from './skills';
 
 // Equipment types