chore: 更新 .gitignore 排除构建产物和临时文件

添加 target/、*.class、__pycache__/、backend/uploads/、*.png 等规则

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.gitignore

@@ -45,4 +45,21 @@ bower_components/
 
 # Misc
 .DS_Store
-Thumbs.db
+Thumbs.db
+
+# Java / Maven build outputs
+target/
+*.class
+*.jar
+*.war
+
+# Backend uploads
+backend/uploads/
+
+# Python
+__pycache__/
+*.pyc
+*.pyo
+
+# Screenshots (root level)
+*.png