feat: Features架构迁移 - P0和P1部分完成

## 完成的工作

### P0: 基础设施准备
- 创建 features/ 和 shared/ 目录结构
- 迁移共享资源：API基础设施、工具函数、类型定义
- 迁移通用composables：useMoney, useSmsCountdown, usePricingCalculator
- 迁移全局样式文件
- 建立模块化导出系统

### P1.1: 钱包模块 (wallet)
- 迁移 API: wallet.ts
- 迁移 Views: WalletView.vue
- 新增 Composable: useWallet.ts (封装钱包状态管理)
- 更新导入路径到 shared/

### P1.2: 聊天模块 (chats)
- 迁移 API: chats.ts
- 迁移 Views: ChatView, MessagesView (桌面+移动)
- 迁移 Composables: useChatSSE.ts
- 迁移 Components: ChatAttachmentImage.vue
- 更新导入路径到 shared/

## 技术改进
- 修复 shared/composables 导出问题 (default → 命名导出)
- 修复 shared/api/client.ts 类型导入路径
- 建立清晰的模块边界和导出规范

## 文档
- 添加完整的迁移计划文档
- 添加进度跟踪文档

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

frontend/src/shared/composables/useSmsCountdown.ts

@@ -0,0 +1,69 @@
+import { onUnmounted, ref } from "vue";
+import { showToast } from "vant";
+import { sendSmsCode } from "@/api/auth";
+
+export function useSmsCountdown() {
+  const countDown = ref(0);
+  const sending = ref(false);
+  let timer: ReturnType<typeof setInterval> | null = null;
+
+  function startCountDown() {
+    countDown.value = 60;
+    timer = setInterval(() => {
+      countDown.value--;
+      if (countDown.value <= 0) {
+        clearInterval(timer!);
+        timer = null;
+      }
+    }, 1000);
+  }
+
+  onUnmounted(() => {
+    if (timer) {
+      clearInterval(timer);
+      timer = null;
+    }
+  });
+
+  async function handleSendCode(phone: string) {
+    if (!phone.trim()) {
+      showToast({ message: "请输入手机号", icon: "warning-o" });
+      return false;
+    }
+
+    sending.value = true;
+    try {
+      await sendSmsCode(phone);
+      showToast({
+        message: "验证码已发送，请注意查收",
+        icon: "passed",
+      });
+      startCountDown();
+      return true;
+    } catch (error) {
+      showToast({
+        message: readError(error, "验证码发送失败，请稍后重试"),
+        icon: "cross",
+      });
+      return false;
+    } finally {
+      sending.value = false;
+    }
+  }
+
+  function readError(error: unknown, fallback: string) {
+    if (typeof error === "object" && error && "response" in error) {
+      const response = (error as { response?: { data?: { message?: string } } })
+        .response;
+      return response?.data?.message || fallback;
+    }
+    return fallback;
+  }
+
+  return {
+    countDown,
+    sending,
+    handleSendCode,
+    readError,
+  };
+}