feat: redesign UI with pastel theme, dark mode and smooth animations

- Add light/dark/system theme switcher with localStorage persistence
  and automatic prefers-color-scheme detection
- New pastel Instagram-inspired palette: rose accent (#C9445D light,
  #E8687E dark), clean #FAFAFA / #000000 backgrounds
- Replace all hardcoded bg-white / red-* colors with CSS variables
  so every page and modal respects the active theme
- Smooth card hover: translateY elevation + image zoom (no layout shift)
- Micro-animations on action buttons with spring cubic-bezier
- Editorial navbar: 3px accent top bar, uppercase wordmark
- Theme toggle button (sun/moon/monitor) added to navbar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/pages/ProfilePage.tsx

@@ -126,7 +126,7 @@ export default function ProfilePage() {
                 </Link>
               )}
               {isMe && (
-                <button onClick={logout} className="btn-ghost h-8 w-8 text-[var(--fg-tertiary)] hover:text-red-500" title="Выйти">
+                <button onClick={logout} className="btn-ghost h-8 w-8 text-[var(--fg-tertiary)] hover:text-[var(--danger)]" title="Выйти">
                   <LogOut className="h-4 w-4" strokeWidth={1.5} />
                 </button>
               )}
@@ -200,7 +200,7 @@ export default function ProfilePage() {
 
       {showAvatarPicker && createPortal(
         <div className="fixed inset-0 z-[70] bg-black/30 flex items-center justify-center animate-fade-in" onClick={() => setShowAvatarPicker(false)}>
-          <div className="bg-white rounded-2xl w-full max-w-sm p-5 shadow-xl animate-scale-in mx-4" onClick={e => e.stopPropagation()}>
+          <div className="bg-[var(--surface)] rounded-2xl w-full max-w-sm p-5 shadow-xl animate-scale-in mx-4" onClick={e => e.stopPropagation()}>
             <div className="flex items-center justify-between mb-5">
               <h2 className="text-base font-800">Выбрать аватар</h2>
               <button onClick={() => setShowAvatarPicker(false)} className="btn-ghost h-8 w-8 text-[var(--fg-tertiary)]">