feat(03-02): wire collapsible sections into DashboardContent

- Add CATEGORY_TYPES_ALL constant and isOverBudget direction-aware helper
- Derive groupedSections via useMemo (filters empty groups, computes totals)
- Initialize openSections state with smart defaults (over-budget expanded)
- State resets on month navigation via key={budgetId} on DashboardContent
- Insert CollapsibleSections between chart grid and QuickAdd
- Add skeleton placeholders for collapsible sections area in DashboardSkeleton

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/dashboard/DashboardSkeleton.tsx

@@ -52,6 +52,21 @@ export function DashboardSkeleton() {
           </CardContent>
         </Card>
       </div>
+
+      {/* Collapsible sections skeleton */}
+      <div className="space-y-3">
+        {[1, 2, 3].map((i) => (
+          <div key={i} className="flex items-center gap-3 rounded-md border-l-4 border-muted bg-card px-4 py-3">
+            <Skeleton className="size-4" />
+            <Skeleton className="h-4 w-32" />
+            <div className="ml-auto flex items-center gap-2">
+              <Skeleton className="h-5 w-24 rounded-full" />
+              <Skeleton className="h-5 w-24 rounded-full" />
+              <Skeleton className="h-4 w-16" />
+            </div>
+          </div>
+        ))}
+      </div>
     </div>
   )
 }