refactor: consolidate formatting utilities and add testing infrastructure

- Replace 8 duplicate formatCurrency functions with shared utility
- Add comprehensive formatter utilities (currency, date, number, etc.)
- Set up Vitest for unit testing with 38 passing tests
- Set up Playwright for E2E testing
- Consolidate database connection to single source (src/utils/db.ts)
- Add auth middleware helpers to reduce code duplication
- Fix display bug: remove spurious minus sign in recent activity amounts
- Add path aliases for cleaner imports ($utils, $models)
- Add project documentation (CODEMAP.md, REFACTORING_PLAN.md)

Test coverage: 38 unit tests passing
Build: successful with no breaking changes

scripts/replace-formatters.md

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+# Formatter Replacement Progress
+
+## Components Completed ✅
+1. DebtBreakdown.svelte - Replaced formatCurrency function
+2. EnhancedBalance.svelte - Replaced formatCurrency function (with Math.abs wrapper)
+
+## Remaining Files to Update
+
+### Components (3 files)
+- [ ] PaymentModal.svelte - Has formatCurrency function
+- [ ] SplitMethodSelector.svelte - Has inline .toFixed() calls
+- [ ] BarChart.svelte - Has inline .toFixed() calls
+- [ ] IngredientsPage.svelte - Has .toFixed() for recipe calculations
+
+### Cospend Pages (7 files)
+- [ ] routes/cospend/+page.svelte - Has formatCurrency function
+- [ ] routes/cospend/payments/+page.svelte - Has formatCurrency function
+- [ ] routes/cospend/payments/view/[id]/+page.svelte - Has formatCurrency and .toFixed()
+- [ ] routes/cospend/payments/add/+page.svelte - Has .toFixed() and .toLocaleString()
+- [ ] routes/cospend/payments/edit/[id]/+page.svelte - Has multiple .toFixed() calls
+- [ ] routes/cospend/recurring/+page.svelte - Has formatCurrency function
+- [ ] routes/cospend/recurring/edit/[id]/+page.svelte - Has .toFixed() and .toLocaleString()
+- [ ] routes/cospend/settle/+page.svelte - Has formatCurrency function
+
+## Replacement Strategy
+
+### Pattern 1: Identical formatCurrency functions
+```typescript
+// OLD
+function formatCurrency(amount) {
+  return new Intl.NumberFormat('de-CH', {
+    style: 'currency',
+    currency: 'CHF'
+  }).format(amount);
+}
+
+// NEW
+import { formatCurrency } from '$lib/utils/formatters';
+// Usage: formatCurrency(amount, 'CHF', 'de-CH')
+```
+
+### Pattern 2: .toFixed() for currency display
+```typescript
+// OLD
+{payment.amount.toFixed(2)}
+
+// NEW
+import { formatNumber } from '$lib/utils/formatters';
+{formatNumber(payment.amount, 2, 'de-CH')}
+```
+
+### Pattern 3: .toLocaleString() for dates
+```typescript
+// OLD
+nextDate.toLocaleString('de-CH', { weekday: 'long', ... })
+
+// NEW
+import { formatDateTime } from '$lib/utils/formatters';
+formatDateTime(nextDate, 'de-CH', { weekday: 'long', ... })
+```
+
+### Pattern 4: Exchange rate display (4 decimals)
+```typescript
+// OLD
+{exchangeRate.toFixed(4)}
+
+// NEW
+{formatNumber(exchangeRate, 4, 'de-CH')}
+```