fix: improve registration error messages with specific validation feedback

Extract PocketBase validation errors (duplicate email/username/password)
and display them in Chinese. Also improve login error message.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/stores/user.ts

@@ -42,7 +42,7 @@ export const useUserStore = defineStore('user', () => {
       await pb.collection('users').authWithPassword(email, password)
       await initUser()
     } catch (error: any) {
-      throw new Error(error.message || '登录失败')
+      throw new Error('邮箱或密码错误')
     } finally {
       loading.value = false
     }
@@ -55,6 +55,14 @@ export const useUserStore = defineStore('user', () => {
       await pb.collection('users').create(data)
       await login(data.email, data.password)
     } catch (error: any) {
+      const validation = error?.data?.data || error?.data
+      if (validation) {
+        const messages: string[] = []
+        if (validation.email) messages.push('邮箱无效或已被使用')
+        if (validation.username) messages.push('用户名无效或已被使用')
+        if (validation.password) messages.push('密码不符合要求')
+        if (messages.length > 0) throw new Error(messages.join('；'))
+      }
       throw new Error(error.message || '注册失败')
     } finally {
       loading.value = false