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/UploadPage.tsx

@@ -51,7 +51,7 @@ export default function UploadPage() {
 
       {!preview ? (
         <div {...getRootProps()}
-          className={`border-2 border-dashed rounded-2xl p-16 text-center cursor-pointer transition-all bg-white ${
+          className={`border-2 border-dashed rounded-2xl p-16 text-center cursor-pointer transition-all bg-[var(--surface)] ${
             isDragActive ? 'border-[var(--accent)] bg-[var(--accent-light)] scale-[1.01]' : 'border-[var(--border)] hover:border-[var(--fg-tertiary)] hover:bg-[var(--surface-hover)]'
           }`}>
           <input {...getInputProps()} />
@@ -110,7 +110,7 @@ export default function UploadPage() {
           </div>
 
           {uploadMutation.isError && (
-            <div className="flex items-center gap-2 px-4 py-3 rounded-xl bg-red-50 text-red-600 text-xs font-medium animate-fade-up">
+            <div className="flex items-center gap-2 px-4 py-3 rounded-xl bg-[var(--danger-light)] text-[var(--danger)] text-xs font-medium animate-fade-up">
               Не удалось загрузить. Попробуйте снова.
             </div>
           )}