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/pages/DashboardPage.tsx

@@ -1,4 +1,4 @@
-import { useMemo } from "react"
+import { useState, useMemo } from "react"
 import { useTranslation } from "react-i18next"
 import { useBudgets, useBudgetDetail } from "@/hooks/useBudgets"
 import { useMonthParam } from "@/hooks/useMonthParam"
@@ -13,6 +13,7 @@ import { MonthNavigator } from "@/components/dashboard/MonthNavigator"
 import { ExpenseDonutChart } from "@/components/dashboard/charts/ExpenseDonutChart"
 import { IncomeBarChart } from "@/components/dashboard/charts/IncomeBarChart"
 import { SpendBarChart } from "@/components/dashboard/charts/SpendBarChart"
+import { CollapsibleSections } from "@/components/dashboard/CollapsibleSections"
 import QuickAddPicker from "@/components/QuickAddPicker"
 
 // ---------------------------------------------------------------------------
@@ -27,6 +28,26 @@ const EXPENSE_TYPES: CategoryType[] = [
   "investment",
 ]
 
+const CATEGORY_TYPES_ALL: CategoryType[] = [
+  "income",
+  "bill",
+  "variable_expense",
+  "debt",
+  "saving",
+  "investment",
+]
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+function isOverBudget(type: CategoryType, budgeted: number, actual: number): boolean {
+  if (type === "income" || type === "saving" || type === "investment") {
+    return actual < budgeted  // under-earned / under-saved
+  }
+  return actual > budgeted    // overspent
+}
+
 // ---------------------------------------------------------------------------
 // Dashboard inner — rendered once a budget id is known
 // ---------------------------------------------------------------------------
@@ -114,6 +135,33 @@ function DashboardContent({ budgetId }: { budgetId: string }) {
     [items, t]
   )
 
+  const groupedSections = useMemo(() =>
+    CATEGORY_TYPES_ALL
+      .map((type) => {
+        const groupItems = items.filter((i) => i.category?.type === type)
+        if (groupItems.length === 0) return null
+        const budgeted = groupItems.reduce((s, i) => s + i.budgeted_amount, 0)
+        const actual = groupItems.reduce((s, i) => s + i.actual_amount, 0)
+        return {
+          type,
+          label: t(`categories.types.${type}`),
+          items: groupItems,
+          budgeted,
+          actual,
+        }
+      })
+      .filter((g): g is NonNullable<typeof g> => g !== null),
+    [items, t]
+  )
+
+  const [openSections, setOpenSections] = useState<Record<string, boolean>>(() =>
+    Object.fromEntries(
+      groupedSections.map((g) => [g.type, isOverBudget(g.type, g.budgeted, g.actual)])
+    )
+  )
+  // Note: state resets automatically on month navigation because DashboardContent
+  // is keyed by budgetId in DashboardPage (key={budgetId} causes full remount)
+
   // ------------------------------------------------------------------
   // Early returns after all hooks
   // ------------------------------------------------------------------
@@ -121,6 +169,10 @@ function DashboardContent({ budgetId }: { budgetId: string }) {
   if (loading) return <DashboardSkeleton />
   if (!budget) return null
 
+  const handleToggleSection = (type: string, open: boolean) => {
+    setOpenSections((prev) => ({ ...prev, [type]: open }))
+  }
+
   const currency = budget.currency
   const availableBalance = totalIncome - totalExpenses + budget.carryover_amount
 
@@ -194,6 +246,17 @@ function DashboardContent({ budgetId }: { budgetId: string }) {
         </Card>
       </div>
 
+      {/* Collapsible category sections */}
+      {groupedSections.length > 0 && (
+        <CollapsibleSections
+          groups={groupedSections}
+          currency={currency}
+          openSections={openSections}
+          onToggleSection={handleToggleSection}
+          t={t}
+        />
+      )}
+
       {/* Quick Add button */}
       <div className="flex justify-end">
         <QuickAddPicker budgetId={budgetId} />
@@ -264,7 +327,7 @@ export default function DashboardPage() {
           </div>
         </div>
       ) : (
-        <DashboardContent budgetId={currentBudget.id} />
+        <DashboardContent key={currentBudget.id} budgetId={currentBudget.id} />
       )}
     </PageShell>
   )