feat: UI 清新风全面改造 — 朝露主题 + shadcn_ui + 全局字号放大

- 全新"朝露"主题：深青绿主色(#2D6A4F)，暖白底，完整设计Token
- 引入 shadcn_ui 组件库，MaterialApp 注入 ShadTheme
- 新增 4 个公共组件：AppCard、AppMenuItem、AppEmptyState、AppTabChip
- 全面消除硬编码颜色，统一使用 AppTheme Token
- 全局字号 +3~4pt，图标等比放大 +3px
- home/medication/profile/settings/login 等核心页面重写
- 路由和功能逻辑零改动

health_app/lib/widgets/app_empty_state.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/material.dart';
+import 'package:shadcn_ui/shadcn_ui.dart';
+
+/// 统一空状态占位组件
+class AppEmptyState extends StatelessWidget {
+  final IconData icon;
+  final String title;
+  final String? subtitle;
+  final Widget? action;
+
+  const AppEmptyState({
+    super.key,
+    required this.icon,
+    required this.title,
+    this.subtitle,
+    this.action,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = ShadTheme.of(context);
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.all(48),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Container(
+              width: 80,
+              height: 80,
+              decoration: BoxDecoration(
+                color: theme.colorScheme.muted,
+                borderRadius: BorderRadius.circular(40),
+              ),
+              child: Icon(icon, size: 36, color: theme.colorScheme.mutedForeground),
+            ),
+            const SizedBox(height: 20),
+            Text(title, style: TextStyle(fontSize: 20, fontWeight: FontWeight.w600, color: theme.colorScheme.foreground)),
+            if (subtitle != null) ...[
+              const SizedBox(height: 6),
+              Text(subtitle!, style: TextStyle(fontSize: 17, color: theme.colorScheme.mutedForeground), textAlign: TextAlign.center),
+            ],
+            if (action != null) ...[
+              const SizedBox(height: 20),
+              action!,
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}