feat: Add comprehensive backend validation, toast notifications, and Tokyo Night theme

- Backend: Add server-side validation with go-playground/validator/v10
- Frontend: Add toast notifications for API errors with @zerodevx/svelte-toast
- UI: Complete Tokyo Night theme redesign with modern animations
- Docs: Update COMPLETE_DOCUMENTATION.md and README.md with all enhancements
- Validation: Email format, password strength, and input sanitization
- UX: Real-time error feedback, loading states, and responsive design

frontend/tailwind.config.js

@@ -0,0 +1,45 @@
+/** @type {import('tailwindcss').Config} */
+export default {
+  content: ['./src/**/*.{html,js,svelte,ts}'],
+  theme: {
+    extend: {
+      colors: {
+        tokyo: {
+          bg: '#1a1b26',
+          'bg-dark': '#16161e',
+          'bg-highlight': '#292e42',
+          'bg-selection': '#364a82',
+          fg: '#a9b1d6',
+          'fg-dark': '#565f89',
+          'fg-light': '#c0caf5',
+          red: '#f7768e',
+          orange: '#ff9e64',
+          yellow: '#e0af68',
+          green: '#9ece6a',
+          cyan: '#7dcfff',
+          blue: '#7aa2f7',
+          purple: '#bb9af7',
+          magenta: '#c0caf5',
+        },
+      },
+      fontFamily: {
+        sans: ['Inter', 'system-ui', 'sans-serif'],
+      },
+      animation: {
+        'fade-in': 'fadeIn 0.5s ease-in-out',
+        'slide-in': 'slideIn 0.3s ease-out',
+      },
+      keyframes: {
+        fadeIn: {
+          '0%': { opacity: '0' },
+          '100%': { opacity: '1' },
+        },
+        slideIn: {
+          '0%': { transform: 'translateY(-10px)', opacity: '0' },
+          '100%': { transform: 'translateY(0)', opacity: '1' },
+        },
+      },
+    },
+  },
+  plugins: [],
+}