Add sales tracking system with customers, analytics, and inventory management

- Add customers table (021) and sales/sale_items tables (022) migrations
- Add customer CRUD, sale CRUD (transactional with auto inventory decrement/restore),
  and analytics API endpoints (overview, top sellers, revenue chart, inventory alerts)
- Add sales page with NewSaleModal (customer autocomplete, multi-item form,
  color-based pricing, stock validation) and SaleDetailModal
- Add customers page with search, inline editing, and purchase history
- Add analytics dashboard with recharts (revenue line chart, top sellers bar,
  refill vs spulna pie chart, inventory alerts table with stockout estimates)
- Add customerService, saleService, analyticsService to frontend API layer
- Update sidebar navigation on all admin pages

database/migrations/021_add_customers_table.sql

@@ -0,0 +1,16 @@
+-- Add customers table for tracking buyers
+CREATE EXTENSION IF NOT EXISTS "pgcrypto";
+
+CREATE TABLE customers (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    name VARCHAR(255) NOT NULL,
+    phone VARCHAR(50),
+    city VARCHAR(255),
+    notes TEXT,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Phone is the natural dedup key
+CREATE UNIQUE INDEX idx_customers_phone ON customers (phone) WHERE phone IS NOT NULL;
+CREATE INDEX idx_customers_name ON customers (name);