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/reset.css

@@ -1,5 +1,5 @@
 /* Reset and normalize styles */
-* {
+*, *::before, *::after {
     margin: 0;
     padding: 0;
     box-sizing: border-box;
@@ -10,7 +10,8 @@ body {
     font-family: var(--font-family-primary);
     background: var(--bg-darkest);
     color: var(--text-primary);
-    height: 100vh;
+    min-height: 100vh;
+    min-height: 100dvh; /* Dynamic viewport height for mobile */
     overflow: hidden;
     position: relative;
 }