Add Comprehensive Testing Suite

.agent/architecture.md

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+# Architecture
+
+## Data Model (Prisma)
+
+```
+Group (id, name, currency, currencyCode)
+  └── Participant (id, name)
+  └── Expense (id, title, amount, expenseDate, splitMode, isReimbursement)
+        ├── paidBy → Participant
+        ├── paidFor → ExpensePaidFor[] (participantId, shares)
+        ├── Category (id, grouping, name)
+        ├── ExpenseDocument[] (url, width, height)
+        └── RecurringExpenseLink (nextExpenseCreatedAt)
+  └── Activity (time, activityType, data) - audit log
+```
+
+### Split Modes
+
+- `EVENLY`: Divide equally, `shares` = 1 per participant
+- `BY_SHARES`: Proportional, e.g., shares 2:1:1 = 50%:25%:25%
+- `BY_PERCENTAGE`: Basis points (10000 = 100%), e.g., 2500 = 25%
+- `BY_AMOUNT`: Direct cents, `shares` = exact amount owed
+
+### Calculations (src/lib/balances.ts)
+
+```typescript
+// BY_PERCENTAGE: (expense.amount * shares) / 10000
+// BY_SHARES: (expense.amount * shares) / totalShares
+// BY_AMOUNT: shares directly
+// Rounding: Math.round() at the end
+```
+
+## Directory Details
+
+### src/app/
+
+Next.js App Router. Pages, layouts, Server Actions. Group pages under `groups/[groupId]/`.
+
+### src/components/
+
+Reusable components. shadcn/UI primitives in `ui/`. Feature components at root.
+
+### src/trpc/
+
+- `init.ts` - tRPC config, SuperJSON transformer
+- `routers/_app.ts` - Root router composition
+- `routers/groups/` - Group domain (expenses, balances, stats, activities)
+- `routers/categories/` - Category CRUD
+
+### src/lib/
+
+- `api.ts` - Database operations (createExpense, updateExpense, etc.)
+- `balances.ts` - Balance calculation logic
+- `totals.ts` - Expense total calculations
+- `schemas.ts` - Zod validation schemas
+- `prisma.ts` - Prisma client singleton
+- `featureFlags.ts` - Feature toggles (S3 docs, receipt scanning)
+
+## tRPC Router Hierarchy
+
+```
+appRouter
+├── groups
+│   ├── get, getDetails, list, create, update
+│   ├── expenses (list, get, create, update, delete)
+│   ├── balances (list)
+│   ├── stats (get)
+│   └── activities (list)
+└── categories
+    └── list
+```
+
+API calls: `trpc.groups.expenses.create()`, `trpc.groups.balances.list()`, etc.
+
+## Feature Flags
+
+Env vars for optional features:
+
+- `NEXT_PUBLIC_ENABLE_EXPENSE_DOCUMENTS` - S3 image uploads
+- `NEXT_PUBLIC_ENABLE_RECEIPT_EXTRACT` - GPT-4V receipt scanning
+- `NEXT_PUBLIC_ENABLE_CATEGORY_EXTRACT` - AI category suggestions

.agent/database.md

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+# Database
+
+## Setup
+
+```bash
+./scripts/start-local-db.sh  # Start PostgreSQL container
+npx prisma migrate dev       # Run migrations
+npx prisma studio            # GUI for database
+npx prisma generate          # Regenerate client after schema changes
+```
+
+## Prisma Client Singleton
+
+```typescript
+// src/lib/prisma.ts
+import { PrismaClient } from '@prisma/client'
+
+const globalForPrisma = global as unknown as { prisma: PrismaClient }
+
+export const prisma = globalForPrisma.prisma || new PrismaClient()
+
+if (process.env.NODE_ENV !== 'production') globalForPrisma.prisma = prisma
+```
+
+Dev mode uses global singleton to survive hot reload.
+
+## Schema Changes
+
+1. Edit `prisma/schema.prisma`
+2. Run `npx prisma migrate dev --name descriptive_name`
+3. Commit migration file + schema changes together
+
+## Query Patterns
+
+### Create with Relations
+
+```typescript
+// src/lib/api.ts - createExpense
+await prisma.expense.create({
+  data: {
+    groupId,
+    title,
+    amount,
+    paidById: paidBy,
+    splitMode,
+    expenseDate,
+    paidFor: {
+      createMany: {
+        data: paidFor.map(({ participant, shares }) => ({
+          participantId: participant,
+          shares,
+        })),
+      },
+    },
+  },
+})
+```
+
+### Query with Includes
+
+```typescript
+// Expenses with payer and split details
+await prisma.expense.findMany({
+  where: { groupId },
+  include: {
+    paidBy: true,
+    paidFor: { include: { participant: true } },
+    category: true,
+  },
+  orderBy: [{ expenseDate: 'desc' }, { createdAt: 'desc' }],
+})
+```
+
+### Update with Nested Operations
+
+```typescript
+// Update expense and replace paidFor entries
+await prisma.expense.update({
+  where: { id: expenseId },
+  data: {
+    title,
+    amount,
+    paidFor: {
+      deleteMany: {}, // Remove all existing
+      createMany: { data: newPaidFor },
+    },
+  },
+})
+```
+
+## Transactions
+
+Used for atomic operations:
+
+```typescript
+// src/lib/api.ts - createRecurringExpenses
+await prisma.$transaction(async (tx) => {
+  const expense = await tx.expense.create({ data: expenseData })
+  await tx.recurringExpenseLink.update({
+    where: { id: linkId },
+    data: { nextExpenseCreatedAt: nextDate },
+  })
+  return expense
+})
+```
+
+## Amount Storage
+
+All monetary values stored as **integers in cents**:
+
+- `100` = $1.00
+- `15050` = $150.50
+
+Split shares vary by mode:
+
+- `EVENLY`: 1 per participant
+- `BY_SHARES`: Weight integers (1, 2, 3...)
+- `BY_PERCENTAGE`: Basis points (2500 = 25%)
+- `BY_AMOUNT`: Cents directly

.agent/testing.md

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+# Testing
+
+## Jest Unit Tests
+
+```bash
+npm test                          # Run all tests
+npm test -- --watch               # Watch mode
+npm test -- path/to/file.test.ts  # Specific file
+```
+
+Tests in `src/**/*.test.ts` alongside implementation.
+
+### Test Data Factory Pattern
+
+```typescript
+// src/lib/balances.test.ts
+const makeExpense = (overrides: Partial<BalancesExpense>): BalancesExpense =>
+  ({
+    id: 'e1',
+    expenseDate: new Date('2025-01-01T00:00:00.000Z'),
+    title: 'Dinner',
+    amount: 0,
+    isReimbursement: false,
+    splitMode: 'EVENLY',
+    paidBy: { id: 'p0', name: 'P0' },
+    paidFor: [{ participant: { id: 'p0', name: 'P0' }, shares: 1 }],
+    ...overrides,
+  }) as BalancesExpense
+
+// Usage
+const expenses = [
+  makeExpense({
+    amount: 100,
+    paidBy: { id: 'p0', name: 'P0' },
+    paidFor: [
+      { participant: { id: 'p0', name: 'P0' }, shares: 1 },
+      { participant: { id: 'p1', name: 'P1' }, shares: 1 },
+    ],
+  }),
+]
+```
+
+### Focus Areas
+
+- `balances.test.ts` - Balance calculations, split modes, edge cases
+- `totals.test.ts` - Expense totals, user shares
+- `currency.test.ts` - Currency formatting
+
+## Playwright E2E Tests
+
+```bash
+npm run test-e2e  # Runs against local dev server
+```
+
+Tests in `tests/e2e/*.spec.ts` and `tests/*.spec.ts`.
+
+### Test Helpers (`tests/helpers/`)
+
+| Helper                                          | Purpose                  |
+| ----------------------------------------------- | ------------------------ |
+| `createGroupViaAPI(page, name, participants)`   | Fast group setup via API |
+| `createExpense(page, { title, amount, payer })` | Fill expense form        |
+| `navigateToExpenseCreate(page, groupId)`        | Go to expense creation   |
+| `fillParticipants(page, names)`                 | Add participants to form |
+| `selectComboboxOption(page, label, value)`      | Select dropdown value    |
+
+### Stability Patterns
+
+```typescript
+// Wait after navigation
+await page.goto(`/groups/${groupId}`)
+await page.waitForLoadState()
+
+// Wait for URL after form submission
+await page.getByRole('button', { name: 'Create' }).click()
+await page.waitForURL(/\/groups\/[^/]+\/expenses/)
+
+// Use API for fast setup
+const groupId = await createGroupViaAPI(page, 'Test Group', ['Alice', 'Bob'])
+```
+
+### Example Test
+
+```typescript
+import { createExpense } from '../helpers'
+import { createGroupViaAPI } from '../helpers/batch-api'
+
+test('creates expense with correct values', async ({ page }) => {
+  const groupId = await createGroupViaAPI(page, `Test ${randomId(4)}`, [
+    'Alice',
+    'Bob',
+  ])
+  await page.goto(`/groups/${groupId}/expenses`)
+
+  await createExpense(page, {
+    title: 'Dinner',
+    amount: '150.00',
+    payer: 'Alice',
+  })
+
+  await expect(page.getByText('Dinner')).toBeVisible()
+  await expect(page.getByText('$150.00')).toBeVisible()
+})
+```
+
+### Config Notes
+
+- `fullyParallel: false` in playwright.config.ts prevents DB conflicts
+- Runs Chromium, Firefox, WebKit
+- `json` reporter when `CLAUDE_CODE` env var detected