fix(ui): resolve Dialog/Modal not opening on first click

Two bugs were caused by ref synchronization race condition in commit 7d495aa:
- EditProviderDialog: provider prop was null on first render
- UsageScriptModal: conditional render guard was false on first click

Root cause: useEffect updates ref asynchronously, but render happens before
effect runs. On first click, ref is still null causing components to fail.

Solution: Create useLastValidValue hook that updates ref synchronously during
render phase instead of in useEffect. This ensures ref is always in sync with
state, eliminating the race condition.

Changes:
- Add useLastValidValue hook for preserving last valid value during animations
- Replace manual ref + useEffect pattern with the new hook
- Remove non-null assertions (!) that were needed as workaround

src/hooks/useLastValidValue.ts

@@ -0,0 +1,20 @@
+import { useRef } from "react";
+
+/**
+ * 保存最后一个非 null/undefined 的值
+ * 用于 Dialog 关闭动画期间保持内容显示
+ *
+ * @param value 当前值
+ * @returns 当前值（如果有效）或最后一个有效值
+ */
+export function useLastValidValue<T>(value: T | null | undefined): T | null {
+  const ref = useRef<T | null>(null);
+
+  // 同步更新 ref（在渲染期间，不在 useEffect 中）
+  if (value != null) {
+    ref.current = value;
+  }
+
+  // 返回当前值或最后有效值
+  return value ?? ref.current;
+}