fix: 返回动画使用 snapTo 消除残留过渡

预测性返回动画结束后直接 snapTo(0f)，避免 spring 动画残留。

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

shared/src/commonMain/kotlin/plus/rua/project/App.kt

@@ -101,7 +101,7 @@ fun App() {
                 Screen.Licenses -> Screen.About
                 else -> currentScreen
             }
-            backAnimProgress.animateTo(0f, spring(stiffness = Spring.StiffnessMedium, dampingRatio = Spring.DampingRatioNoBouncy))
+            backAnimProgress.snapTo(0f)
             isHandlingBack = false
         }
     }