Add comprehensive unit tests for Security, UserModel, and Validation utilities

- Implemented SecurityTest to validate token generation, CSRF protection, input sanitization, and rate limiting.
- Created UserModelTest to ensure correct database operations for user management, including creation, updating, banning, and fetching active users.
- Developed ValidationTest to verify input validation and sanitization for user IDs, nicknames, messages, and API requests.
- Introduced Security and Validation utility classes with methods for secure token generation, input sanitization, and comprehensive validation rules.

static/css/variables.css

@@ -114,6 +114,7 @@
     /* Borders & Dividers */
     --border-color: #1a2332;
     --border-color-light: #232d3a;
+    --border-color-humane: #232d3a; /* For responsive borders */
     --divider-color: rgba(255, 255, 255, 0.1);
 
     /* Input & Form Elements */
@@ -303,17 +304,34 @@
     --mq-dashboard-enabled: (min-width: var(--breakpoint-2xl));
 
     /* =================================================================
-       Z-INDEX SCALE
+       Z-INDEX SCALE - Systematic Layer System
        ================================================================= */
 
-    --z-dropdown: 1000;
-    --z-sticky: 1020;
-    --z-fixed: 1030;
-    --z-modal-backdrop: 1040;
-    --z-modal: 1050;
-    --z-popover: 1060;
-    --z-tooltip: 1070;
-    --z-toast: 1080;
+    /* Base layers (0-99) */
+    --z-base: 0;
+    --z-ground: 1;
+
+    /* Content layers (100-299) */
+    --z-content: 100;
+    --z-card: 110;
+    --z-overlay: 200;
+
+    /* Fixed/Positioned elements (300-599) */
+    --z-sticky: 300;
+    --z-dropdown: 400;
+    --z-fixed: 500;
+
+    /* Modals/Dialogs (600-799) */
+    --z-modal-backdrop: 600;
+    --z-modal: 700;
+    --z-popover: 750;
+
+    /* Notifications/Feedback (800-999) */
+    --z-tooltip: 800;
+    --z-toast: 900;
+
+    /* Legacy compatibility */
+    --z-modal-legacy: 1000; /* Keep for existing code */
 
     /* =================================================================
        ICON SIZE SCALE