feat: add bottom navigation shell with study/manage/stats/account tabs

lib/widgets/empty_state.dart

@@ -0,0 +1,51 @@
+import 'package:flutter/material.dart';
+
+/// 各主页面共用的占位内容：一个图标、一句标题和可选说明。
+///
+/// 页面接上真实业务后可以直接替换掉。
+class EmptyState extends StatelessWidget {
+  const EmptyState({
+    required this.icon,
+    required this.title,
+    this.message,
+    super.key,
+  });
+
+  final IconData icon;
+  final String title;
+  final String? message;
+
+  @override
+  Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
+    final String? message = this.message;
+
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 32),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: <Widget>[
+            Icon(icon, size: 56, color: theme.colorScheme.primary),
+            const SizedBox(height: 16),
+            Text(
+              title,
+              style: theme.textTheme.titleMedium,
+              textAlign: TextAlign.center,
+            ),
+            if (message != null) ...<Widget>[
+              const SizedBox(height: 8),
+              Text(
+                message,
+                style: theme.textTheme.bodyMedium?.copyWith(
+                  color: theme.colorScheme.onSurfaceVariant,
+                ),
+                textAlign: TextAlign.center,
+              ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}