feat: add collapsible skill categories in SkillsTab

- Add collapsedCategories state to track which categories are collapsed
- Add toggleCategory function to toggle collapse state
- Update CardHeader to be clickable for collapse/expand
- Show ChevronDown/ChevronRight icons based on collapse state
- Show skill count badge in each category header

src/components/game/tabs/SkillsTab.tsx

@@ -14,6 +14,7 @@ import { Badge } from '@/components/ui/badge';
 import { Tooltip, TooltipContent, TooltipProvider, TooltipTrigger } from '@/components/ui/tooltip';
 import { StudyProgress } from './StudyProgress';
 import { UpgradeDialog } from './UpgradeDialog';
+import { ChevronDown, ChevronRight } from 'lucide-react';
 
 export interface SkillsTabProps {
   store: GameStore;
@@ -55,10 +56,24 @@ export function SkillsTab({ store }: SkillsTabProps) {
   const [upgradeDialogSkill, setUpgradeDialogSkill] = useState<string | null>(null);
   const [upgradeDialogMilestone, setUpgradeDialogMilestone] = useState<5 | 10>(5);
   const [pendingUpgradeSelections, setPendingUpgradeSelections] = useState<string[]>([]);
+  const [collapsedCategories, setCollapsedCategories] = useState<Set<string>>(new Set());
   
   const studySpeedMult = getStudySpeedMultiplier(store.skills);
   const upgradeEffects = getUnifiedEffects(store);
   
+  // Toggle category collapse
+  const toggleCategory = (categoryId: string) => {
+    setCollapsedCategories(prev => {
+      const newSet = new Set(prev);
+      if (newSet.has(categoryId)) {
+        newSet.delete(categoryId);
+      } else {
+        newSet.add(categoryId);
+      }
+      return newSet;
+    });
+  };
+  
   // Get upgrade choices for dialog
   const getUpgradeChoices = () => {
     if (!upgradeDialogSkill) return { available: [] as SkillUpgradeChoice[], selected: [] as string[] };
@@ -138,13 +153,20 @@ export function SkillsTab({ store }: SkillsTabProps) {
             const skillsInCat = Object.entries(SKILLS_DEF).filter(([, def]) => def.cat === cat.id);
             if (skillsInCat.length === 0) return null;
             
+            const isCollapsed = collapsedCategories.has(cat.id);
+            
             return (
           <Card key={cat.id} className="bg-gray-900/80 border-gray-700">
-            <CardHeader className="pb-2">
-              <CardTitle className="text-amber-400 game-panel-title text-xs">
-                {cat.icon} {cat.name}
+            <CardHeader className="pb-2 cursor-pointer" onClick={() => toggleCategory(cat.id)}>
+              <CardTitle className="text-amber-400 game-panel-title text-xs flex items-center justify-between">
+                <span>{cat.icon} {cat.name}</span>
+                <div className="flex items-center gap-2">
+                  <Badge variant="outline" className="text-xs">{skillsInCat.length} skills</Badge>
+                  {isCollapsed ? <ChevronRight className="w-4 h-4" /> : <ChevronDown className="w-4 h-4" />}
+                </div>
               </CardTitle>
             </CardHeader>
+            {!isCollapsed && (
             <CardContent>
               <div className="space-y-2">
                 {skillsInCat.map(([id, def]) => {
@@ -337,6 +359,7 @@ export function SkillsTab({ store }: SkillsTabProps) {
                 })}
               </div>
             </CardContent>
+            )}
           </Card>
         );
           });