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>

sources/scss/components/_validation.scss

@@ -0,0 +1,87 @@
+/**
+ * Validation Toast Component
+ * Error notifications for selection conflicts
+ */
+
+@use '../variables' as *;
+@use '../mixins' as *;
+
+// Validation error toast
+.es-validation-toast {
+    display: flex;
+    align-items: flex-start;
+    gap: $es-spacing-sm;
+    padding: $es-spacing-md;
+    background: $es-white;
+    border: 1px solid $es-danger;
+    border-left: 4px solid $es-danger;
+    border-radius: $es-radius-md;
+    box-shadow: $es-shadow-lg;
+    max-width: 400px;
+    animation: es-toast-slide-in 0.2s ease-out;
+
+    .es-toast-icon {
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        width: 24px;
+        height: 24px;
+        color: $es-danger;
+        flex-shrink: 0;
+
+        i {
+            font-size: 18px;
+        }
+    }
+
+    .es-toast-content {
+        flex: 1;
+        min-width: 0;
+    }
+
+    .es-toast-title {
+        font-size: $es-font-size-sm;
+        font-weight: $es-font-weight-semibold;
+        color: $es-danger;
+        margin-bottom: 2px;
+    }
+
+    .es-toast-message {
+        font-size: $es-font-size-xs;
+        color: $es-text-secondary;
+        line-height: 1.4;
+    }
+
+    .es-toast-close {
+        @include button-reset;
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        width: 20px;
+        height: 20px;
+        color: $es-text-muted;
+        border-radius: $es-radius-sm;
+        flex-shrink: 0;
+        transition: all $es-transition-fast;
+
+        &:hover {
+            background: $es-slate-100;
+            color: $es-text-primary;
+        }
+
+        i {
+            font-size: 12px;
+        }
+    }
+}
+
+@keyframes es-toast-slide-in {
+    from {
+        opacity: 0;
+        transform: translateY(-10px);
+    }
+    to {
+        opacity: 1;
+        transform: translateY(0);
+    }
+}