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

src/lib/components/PaymentModal.svelte

@@ -5,7 +5,8 @@
   import ProfilePicture from './ProfilePicture.svelte';
   import EditButton from './EditButton.svelte';
   import { getCategoryEmoji, getCategoryName } from '$lib/utils/categories';
-  
+  import { formatCurrency as formatCurrencyUtil } from '$lib/utils/formatters';
+
   export let paymentId;
   
   // Get session from page store
@@ -63,10 +64,7 @@
   }
 
   function formatCurrency(amount) {
-    return new Intl.NumberFormat('de-CH', {
-      style: 'currency',
-      currency: 'CHF'
-    }).format(Math.abs(amount));
+    return formatCurrencyUtil(Math.abs(amount), 'CHF', 'de-CH');
   }
 
   function formatDate(dateString) {