style(chat): Zmiana sidebara na jasną kolorystykę

- Tło sidebar: #1e1e2e → #f5f7fa (jasny szary)
- Tekst: biały → #374151 (ciemnoszary)
- Border: #2d2d3d → #e5e7eb (jasny)
- Hover/active: białe tło z subtelnym cieniem
- Lepszy kontrast i czytelność

Inspiracja: kolorystyka z wideo demo NordaGPT

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

templates/chat.html

@@ -16,21 +16,21 @@
     }
 
     /* ============================================
-       SIDEBAR - Historia konwersacji
+       SIDEBAR - Historia konwersacji (jasna wersja)
        ============================================ */
     .chat-sidebar {
         width: 280px;
-        background: #1e1e2e;
-        color: white;
+        background: #f5f7fa;
+        color: #374151;
         display: flex;
         flex-direction: column;
-        border-right: 1px solid #2d2d3d;
+        border-right: 1px solid #e5e7eb;
         flex-shrink: 0;
     }
 
     .sidebar-header {
         padding: var(--spacing-md);
-        border-bottom: 1px solid #2d2d3d;
+        border-bottom: 1px solid #e5e7eb;
     }
 
     .new-chat-btn {
@@ -65,7 +65,7 @@
         font-size: var(--font-size-xs);
         text-transform: uppercase;
         letter-spacing: 0.1em;
-        color: #888;
+        color: #6b7280;
         font-weight: 600;
     }
 
@@ -87,17 +87,18 @@
     }
 
     .conversation-item:hover {
-        background: #2d2d3d;
+        background: #ffffff;
     }
 
     .conversation-item.active {
-        background: #3d3d4d;
+        background: #ffffff;
+        box-shadow: 0 1px 3px rgba(0,0,0,0.08);
     }
 
     .conversation-item svg {
         width: 16px;
         height: 16px;
-        opacity: 0.7;
+        color: #9ca3af;
         flex-shrink: 0;
     }
 
@@ -113,7 +114,7 @@
         opacity: 0;
         background: none;
         border: none;
-        color: #888;
+        color: #9ca3af;
         cursor: pointer;
         padding: 4px;
         border-radius: var(--radius-sm);
@@ -132,14 +133,14 @@
     .sidebar-empty {
         padding: var(--spacing-lg);
         text-align: center;
-        color: #666;
+        color: #6b7280;
         font-size: var(--font-size-sm);
     }
 
     .sidebar-loading {
         padding: var(--spacing-lg);
         text-align: center;
-        color: #666;
+        color: #6b7280;
     }
 
     /* ============================================
@@ -418,6 +419,7 @@
             bottom: 0;
             z-index: 100;
             width: 280px;
+            box-shadow: 4px 0 20px rgba(0,0,0,0.1);
         }
 
         .chat-sidebar.mobile-open {