feat(01-03): add data hooks, utilities, UI store, and foundational components

- API fetch wrapper with error handling and multipart upload
- Weight/price formatters for display
- TanStack Query hooks for items, categories, and totals with cache invalidation
- Zustand UI store for panel and confirm dialog state
- TotalsBar, CategoryHeader, ItemCard, ConfirmDialog, ImageUpload components

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/lib/formatters.ts

@@ -0,0 +1,9 @@
+export function formatWeight(grams: number | null | undefined): string {
+  if (grams == null) return "--";
+  return `${Math.round(grams)}g`;
+}
+
+export function formatPrice(cents: number | null | undefined): string {
+  if (cents == null) return "--";
+  return `$${(cents / 100).toFixed(2)}`;
+}