fix: 移除所有金币硬编码，改为API操作

- mine.vue: coins默认值改为0
- coins.vue: 添加API调用更新金币
- 新增addCoins API接口用于金币奖励
- 观看视频奖励通过API更新数据库

src/main/java/xuan/controller/UserController.java

@@ -293,4 +293,30 @@ public class UserController {
         }
         return result;
     }
+    
+    @PostMapping("/addCoins")
+    public Map<String, Object> addCoins(@RequestBody Map<String, Object> params) {
+        Map<String, Object> result = new HashMap<>();
+        try {
+            String userId = params.get("userId").toString();
+            Integer amount = params.get("amount") != null ? Integer.parseInt(params.get("amount").toString()) : 1;
+            String type = params.get("type") != null ? params.get("type").toString() : "unknown";
+            String description = params.get("description") != null ? params.get("description").toString() : "金币奖励";
+            
+            UserInfo user = userInfoService.addCoins(userId, amount, type, description);
+            if (user != null) {
+                Map<String, Object> data = new HashMap<>();
+                data.put("coins", user.getCoins());
+                result.put("success", true);
+                result.put("data", data);
+            } else {
+                result.put("success", false);
+                result.put("error", "用户不存在");
+            }
+        } catch (Exception e) {
+            result.put("success", false);
+            result.put("error", e.getMessage());
+        }
+        return result;
+    }
 }