feat(ui): 底部导航栏添加文字标签

- 在图标下方添加文字标签（首页、行情、交易、资产、我的）
- 激活状态文字加粗
- 支持主题切换
- 提升用户体验和可识别性

flutter_monisuo/lib/ui/pages/main/main_page.dart

@@ -273,7 +273,7 @@ class _NavItemWidget extends StatelessWidget {
       behavior: HitTestBehavior.opaque,
       child: AnimatedContainer(
         duration: const Duration(milliseconds: 300),
-        padding: const EdgeInsets.all(12),
+        padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
         decoration: isSelected
             ? BoxDecoration(
                 color: colorScheme.primary.withOpacity(0.1),
@@ -287,10 +287,24 @@ class _NavItemWidget extends StatelessWidget {
                 ],
               )
             : null,
-        child: Icon(
-          item.icon,
-          color: color,
-          size: 24,
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(
+              item.icon,
+              color: color,
+              size: 24,
+            ),
+            const SizedBox(height: 4),
+            Text(
+              item.label,
+              style: TextStyle(
+                color: color,
+                fontSize: 12,
+                fontWeight: isSelected ? FontWeight.w600 : FontWeight.normal,
+              ),
+            ),
+          ],
         ),
       ),
     );