feat(pos): add 5 quick improvements — dark mode, email quotes, barcode scan, returns, offline catalog

1. Auto dark mode: detect system prefers-color-scheme, auto-switch industrial/modern theme
2. Email quotation endpoint: POST /quotations/:id/email sends HTML email via SMTP
3. Camera barcode scanner: BarcodeDetector API with getUserMedia overlay in catalog
4. Returns with warranty: POST /returns endpoint with stock restoration and sale status tracking
5. Partial offline catalog: cache top 500 parts in IndexedDB, search when offline

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

pos/migrations/v1.5_returns.sql

@@ -0,0 +1,32 @@
+-- v1.5 Returns & warranty support
+-- Applied to each tenant database
+
+CREATE TABLE IF NOT EXISTS returns (
+    id SERIAL PRIMARY KEY,
+    sale_id INTEGER REFERENCES sales(id),
+    customer_id INTEGER REFERENCES customers(id),
+    employee_id INTEGER REFERENCES employees(id),
+    total_refund NUMERIC(12,2) NOT NULL DEFAULT 0,
+    reason TEXT NOT NULL,
+    status VARCHAR(20) NOT NULL DEFAULT 'completed',
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS return_items (
+    id SERIAL PRIMARY KEY,
+    return_id INTEGER REFERENCES returns(id) ON DELETE CASCADE,
+    sale_item_id INTEGER,
+    inventory_id INTEGER REFERENCES inventory(id),
+    quantity INTEGER NOT NULL,
+    unit_price NUMERIC(12,2),
+    refund_amount NUMERIC(12,2)
+);
+
+CREATE INDEX IF NOT EXISTS idx_returns_sale_id ON returns(sale_id);
+CREATE INDEX IF NOT EXISTS idx_returns_customer_id ON returns(customer_id);
+CREATE INDEX IF NOT EXISTS idx_returns_created_at ON returns(created_at);
+CREATE INDEX IF NOT EXISTS idx_return_items_return_id ON return_items(return_id);
+CREATE INDEX IF NOT EXISTS idx_return_items_sale_item_id ON return_items(sale_item_id);
+
+-- Add 'partially_returned' and 'returned' to sales status if not using enum
+-- (sales.status is VARCHAR, so no ALTER TYPE needed)