chore: Android 正式签名配置 + 截图尺寸适配

- 生成 release.jks 正式签名密钥库
- build.gradle.kts 配置 release 签名自动读取 key.properties
- .gitignore 排除 keystore 和 key.properties
- 截图调整为 6.5 寸规格 (1242x2688)

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

.gitignore

@@ -4,6 +4,10 @@
 *.key
 *.pfx
 
+# Android release keystore
+health_app/android/**/*.jks
+health_app/android/key.properties
+
 # .NET build outputs
 backend/**/bin/
 backend/**/obj/