feat: Linear-style UI redesign with improved mobile experience

Major changes:
- Redesign cabinet with Linear-style components and top navigation
- Replace detail modals with dedicated pages (users, broadcasts, email templates)
- Add email channel support for broadcasts
- Remove pull-to-refresh, improve drag-and-drop on touch devices
- Fix Telegram Mini App: fullscreen, back button, scroll restoration
- Unify admin pages color scheme to design system
- Mobile-first improvements: horizontal tabs for settings, better touch targets

src/components/primitives/Button/Button.variants.ts

@@ -0,0 +1,64 @@
+import { cva, type VariantProps } from 'class-variance-authority';
+
+export const buttonVariants = cva(
+  // Base styles
+  [
+    'inline-flex items-center justify-center gap-2',
+    'font-medium transition-all duration-200',
+    'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent-500/50 focus-visible:ring-offset-2 focus-visible:ring-offset-dark-950',
+    'disabled:pointer-events-none disabled:opacity-50',
+    'select-none',
+  ],
+  {
+    variants: {
+      variant: {
+        primary: [
+          'bg-accent-500 text-white',
+          'hover:bg-accent-600',
+          'active:bg-accent-700',
+          'shadow-linear-sm hover:shadow-linear',
+        ],
+        secondary: [
+          'bg-dark-800/80 text-dark-100',
+          'border border-dark-700/50',
+          'hover:bg-dark-700/80 hover:border-dark-600/50',
+          'active:bg-dark-800',
+        ],
+        ghost: ['text-dark-300', 'hover:text-dark-100 hover:bg-dark-800/50', 'active:bg-dark-800'],
+        destructive: [
+          'bg-error-500/10 text-error-400',
+          'border border-error-500/20',
+          'hover:bg-error-500/20 hover:border-error-500/30',
+          'active:bg-error-500/30',
+        ],
+        outline: [
+          'border border-dark-700/50 text-dark-200',
+          'hover:bg-dark-800/50 hover:border-dark-600/50 hover:text-dark-100',
+          'active:bg-dark-800',
+        ],
+        link: [
+          'text-accent-400',
+          'hover:text-accent-300 hover:underline',
+          'active:text-accent-500',
+        ],
+      },
+      size: {
+        sm: 'h-8 px-3 text-sm rounded-linear',
+        md: 'h-10 px-4 text-sm rounded-linear',
+        lg: 'h-12 px-6 text-base rounded-linear-lg',
+        icon: 'h-10 w-10 rounded-linear',
+        'icon-sm': 'h-8 w-8 rounded-linear',
+        'icon-lg': 'h-12 w-12 rounded-linear-lg',
+      },
+      fullWidth: {
+        true: 'w-full',
+      },
+    },
+    defaultVariants: {
+      variant: 'primary',
+      size: 'md',
+    },
+  },
+);
+
+export type ButtonVariants = VariantProps<typeof buttonVariants>;