Add selection validation and improve tooltip component

- Add validation system to prevent contradicting conditions:
  - Same entity in include/exclude detection
  - Parent-child conflict detection for tree entities
  - Redundant selection prevention
  - Toast notifications for validation errors

- Fix entity icons (employees: briefcase, taxes: calculator)

- Improve tooltip component:
  - Use Material Icons instead of broken FA4 icons
  - Fix positioning using getBoundingClientRect for viewport coords
  - Add click-to-pin functionality with close button
  - Pinned tooltips show X icon and close button in corner

- Add lightweight test suite (31 tests) for validation logic

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/EntitySelector.php

@@ -1319,7 +1319,7 @@ trait EntitySelector
             ],
             'suppliers' => [
                 'label' => $this->transEntitySelector('Suppliers'),
-                'icon' => 'icon-truck',
+                'icon' => 'icon-archive',
                 'entity_label' => $this->transEntitySelector('supplier'),
                 'entity_label_plural' => $this->transEntitySelector('suppliers'),
                 'selection_methods' => $this->getSupplierSelectionMethods(),
@@ -1341,21 +1341,21 @@ trait EntitySelector
             // Transactional / System entities
             'employees' => [
                 'label' => $this->transEntitySelector('Employees'),
-                'icon' => 'icon-user-secret',
+                'icon' => 'icon-briefcase',
                 'entity_label' => $this->transEntitySelector('employee'),
                 'entity_label_plural' => $this->transEntitySelector('employees'),
                 'selection_methods' => $this->getEmployeeSelectionMethods(),
             ],
             'customers' => [
                 'label' => $this->transEntitySelector('Customers'),
-                'icon' => 'icon-users',
+                'icon' => 'icon-user',
                 'entity_label' => $this->transEntitySelector('customer'),
                 'entity_label_plural' => $this->transEntitySelector('customers'),
                 'selection_methods' => $this->getCustomerSelectionMethods(),
             ],
             'customer_groups' => [
                 'label' => $this->transEntitySelector('Customer Groups'),
-                'icon' => 'icon-group',
+                'icon' => 'icon-users',
                 'entity_label' => $this->transEntitySelector('customer group'),
                 'entity_label_plural' => $this->transEntitySelector('customer groups'),
                 'selection_methods' => $this->getCustomerGroupSelectionMethods(),
@@ -1418,7 +1418,7 @@ trait EntitySelector
             ],
             'taxes' => [
                 'label' => $this->transEntitySelector('Taxes'),
-                'icon' => 'icon-money',
+                'icon' => 'icon-calculator',
                 'entity_label' => $this->transEntitySelector('tax'),
                 'entity_label_plural' => $this->transEntitySelector('taxes'),
                 'selection_methods' => $this->getTaxSelectionMethods(),