Support rebalance and validate purchase splits

Add ParticipantShare type and client-side rebalance/validation helpers for purchase splits (rebalancePurchaseSplit, recalculatePercentages, calculateRemainingToAllocate, validatePurchaseDraft). Integrate rebalancing and validation into the miniapp ledger UI (auto-adjust shares, percentage<>amount syncing, remaining/error display, prefill participants on new purchase, disable save when invalid). On the backend, tighten input validation in finance-command-service for custom_amounts (require explicit shares and sum match) and make settlement-engine more lenient when reading legacy/malformed custom splits (ignore missing shares, accept explicit subset). Also add a test ensuring dashboard generation doesn't 500 on legacy malformed purchases.

packages/application/src/finance-command-service.test.ts

@@ -752,4 +752,72 @@ describe('createFinanceCommandService', () => {
       }
     ])
   })
+
+  test('generateDashboard should not 500 on legacy malformed custom split purchases (mixed null/explicit shares)', async () => {
+    const repository = new FinanceRepositoryStub()
+    repository.members = [
+      {
+        id: 'alice',
+        telegramUserId: '1',
+        displayName: 'Alice',
+        rentShareWeight: 1,
+        isAdmin: true
+      },
+      {
+        id: 'bob',
+        telegramUserId: '2',
+        displayName: 'Bob',
+        rentShareWeight: 1,
+        isAdmin: false
+      }
+    ]
+    repository.openCycleRecord = {
+      id: 'cycle-2026-03',
+      period: '2026-03',
+      currency: 'GEL'
+    }
+    repository.rentRule = {
+      amountMinor: 70000n,
+      currency: 'USD'
+    }
+    repository.purchases = [
+      {
+        id: 'malformed-purchase-1',
+        payerMemberId: 'alice',
+        amountMinor: 1000n, // Total is 10.00 GEL
+        currency: 'GEL',
+        description: 'Legacy purchase',
+        occurredAt: instantFromIso('2026-03-12T11:00:00.000Z'),
+        splitMode: 'custom_amounts',
+        participants: [
+          {
+            memberId: 'alice',
+            included: true,
+            shareAmountMinor: 1000n // Explicitly Alice takes full 10.00 GEL
+          },
+          {
+            memberId: 'bob',
+            // Missing included: false, and shareAmountMinor is null
+            // This is the malformed data that used to cause 500
+            shareAmountMinor: null
+          }
+        ]
+      }
+    ]
+
+    const service = createService(repository)
+    const dashboard = await service.generateDashboard()
+
+    expect(dashboard).not.toBeNull()
+    const purchase = dashboard?.ledger.find((e) => e.id === 'malformed-purchase-1')
+    expect(purchase?.purchaseSplitMode).toBe('custom_amounts')
+
+    // Bob should be treated as excluded from the settlement calculation
+    const bobLine = dashboard?.members.find((m) => m.memberId === 'bob')
+    expect(bobLine?.purchaseOffset.amountMinor).toBe(0n)
+
+    const aliceLine = dashboard?.members.find((m) => m.memberId === 'alice')
+    // Alice paid 1000n and her share is 1000n -> offset 0n
+    expect(aliceLine?.purchaseOffset.amountMinor).toBe(0n)
+  })
 })

packages/application/src/finance-command-service.ts

@@ -15,6 +15,8 @@ import type {
 import {
   BillingCycleId,
   BillingPeriod,
+  DomainError,
+  DOMAIN_ERROR_CODE,
   MemberId,
   Money,
   PurchaseEntryId,
@@ -867,6 +869,27 @@ export function createFinanceCommandService(
       )
       const currency = parseCurrency(currencyArg, settings.settlementCurrency)
       const amount = Money.fromMajor(amountArg, currency)
+
+      if (split?.mode === 'custom_amounts') {
+        if (split.participants.some((p) => p.shareAmountMajor === undefined)) {
+          throw new DomainError(
+            DOMAIN_ERROR_CODE.INVALID_SETTLEMENT_INPUT,
+            'Purchase custom split must include explicit share amounts for every participant'
+          )
+        }
+
+        const totalMinor = split.participants.reduce(
+          (sum, p) => sum + Money.fromMajor(p.shareAmountMajor!, currency).amountMinor,
+          0n
+        )
+        if (totalMinor !== amount.amountMinor) {
+          throw new DomainError(
+            DOMAIN_ERROR_CODE.INVALID_SETTLEMENT_INPUT,
+            'Purchase custom split must add up to the full amount'
+          )
+        }
+      }
+
       const updated = await repository.updateParsedPurchase({
         purchaseId,
         amountMinor: amount.amountMinor,

packages/application/src/settlement-engine.ts

@@ -259,19 +259,18 @@ export function calculateMonthlySettlement(input: SettlementInput): SettlementRe
     payer.purchasePaid = payer.purchasePaid.add(purchase.amount)
 
     const participants = purchaseParticipantMembers(activeMembers, purchase)
-    const explicitShareAmounts = purchase.participants?.map(
-      (participant) => participant.shareAmount
-    )
 
-    if (explicitShareAmounts && explicitShareAmounts.some((amount) => amount !== undefined)) {
-      if (explicitShareAmounts.some((amount) => amount === undefined)) {
-        throw new DomainError(
-          DOMAIN_ERROR_CODE.INVALID_SETTLEMENT_INPUT,
-          `Purchase custom split must include explicit share amounts for every participant: ${purchase.purchaseId.toString()}`
-        )
-      }
+    // Identify participants with explicit share amounts (lenient read path for legacy data)
+    const explicitShares =
+      purchase.participants
+        ?.filter((p) => p.shareAmount !== undefined)
+        .map((p) => ({
+          memberId: p.memberId,
+          shareAmount: p.shareAmount!
+        })) ?? []
 
-      const shares = explicitShareAmounts as readonly Money[]
+    if (explicitShares.length > 0) {
+      const shares = explicitShares.map((p) => p.shareAmount)
       const shareTotal = sumMoney(shares, currency)
       if (!shareTotal.equals(purchase.amount)) {
         throw new DomainError(
@@ -280,15 +279,13 @@ export function calculateMonthlySettlement(input: SettlementInput): SettlementRe
         )
       }
 
-      for (const [index, member] of participants.entries()) {
-        const state = membersById.get(member.memberId.toString())
+      for (const participant of explicitShares) {
+        const state = membersById.get(participant.memberId.toString())
         if (!state) {
           continue
         }
 
-        state.purchaseSharedCost = state.purchaseSharedCost.add(
-          shares[index] ?? Money.zero(currency)
-        )
+        state.purchaseSharedCost = state.purchaseSharedCost.add(participant.shareAmount)
       }
 
       continue