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/pages/AdminDashboard.tsx

@@ -1,6 +1,5 @@
 import { useState, useEffect } from 'react';
 import { useTranslation } from 'react-i18next';
-import { Link } from 'react-router-dom';
 import {
   statsApi,
   type DashboardStats,
@@ -10,19 +9,9 @@ import {
   type RecentPaymentsResponse,
 } from '../api/admin';
 import { useCurrency } from '../hooks/useCurrency';
+import { AdminBackButton } from '../components/admin';
 
 // Icons - styled like main navigation
-const BackIcon = () => (
-  <svg
-    className="h-5 w-5 text-dark-400"
-    fill="none"
-    viewBox="0 0 24 24"
-    stroke="currentColor"
-    strokeWidth={2}
-  >
-    <path strokeLinecap="round" strokeLinejoin="round" d="M15.75 19.5L8.25 12l7.5-7.5" />
-  </svg>
-);
 
 const ServerIcon = () => (
   <svg className="h-5 w-5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={1.5}>
@@ -461,12 +450,7 @@ export default function AdminDashboard() {
       {/* Header */}
       <div className="flex items-center justify-between">
         <div className="flex items-center gap-3">
-          <Link
-            to="/admin"
-            className="flex h-10 w-10 items-center justify-center rounded-xl border border-dark-700 bg-dark-800 transition-colors hover:border-dark-600"
-          >
-            <BackIcon />
-          </Link>
+          <AdminBackButton />
           <div>
             <h1 className="text-2xl font-bold text-dark-100">{t('adminDashboard.title')}</h1>
             <p className="text-dark-400">{t('adminDashboard.subtitle')}</p>