Fix nav transparency: flat-cards rule was zeroing nav bg+shadow

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

index.html

@@ -283,7 +283,7 @@ body {
 /* Карточки — флэт, без скруглений и теней */
 .card, .tool-card, .create-card, .plan-step, .gloss-item,
 .project-card, .prompt-card, .eq-box, .check-card, .ba-card,
-main, nav {
+main {
   border-radius: 0 !important;
   box-shadow: none !important;
   background: transparent !important;