Add interactive customer selection to checkout modal

Replaced static customer display in the checkout/sale modal with the same interactive button functionality from the cart sidebar.

Changes:
- **HTML**: Replaced static div with interactive button + reset button in checkout modal
- **JavaScript**:
  - Updated updateCustomerDisplay() to handle both locations (cart + checkout modal)
  - Added event listeners for checkout modal customer buttons
  - Both buttons now synchronized and use the same selection modal

Benefits:
✅ Consistent UX across cart and checkout modal
✅ Full code reuse - same selection modal, search, and logic
✅ Both locations stay synchronized automatically
✅ Can search, select, and reset customer directly from checkout modal

Implementation:
- Cart sidebar button: customerSelectBtn, resetCustomerBtn
- Checkout modal button: checkoutCustomerSelectBtn, checkoutResetCustomerBtn
- Single updateCustomerDisplay() updates both locations
- Single selectCustomer() used by all buttons

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

myproject/pos/templates/pos/terminal.html

@@ -277,9 +277,18 @@
           <div class="col-md-7">
             <!-- Информация о клиенте -->
             <div class="mb-3">
-              <strong>Клиент</strong>
-              <div class="border rounded p-2 mt-2 bg-light">
-                <div class="fw-bold" id="checkoutCustomerName">—</div>
+              <label class="form-label fw-semibold">Клиент</label>
+              <div class="d-flex gap-1 align-items-center">
+                <button class="btn btn-outline-primary btn-sm d-flex align-items-center" id="checkoutCustomerSelectBtn">
+                  <i class="bi bi-person me-1"></i>
+                  <div class="d-flex flex-column align-items-start lh-1">
+                    <small class="text-muted" style="font-size: 0.65rem;">Клиент</small>
+                    <span id="checkoutCustomerSelectBtnText" class="fw-semibold">Выбрать</span>
+                  </div>
+                </button>
+                <button class="btn btn-sm btn-outline-danger" id="checkoutResetCustomerBtn" title="Сброс на системного клиента" style="display: none;">
+                  <i class="bi bi-x-lg"></i>
+                </button>
               </div>
             </div>