安全区域改为从 key window 动态获取

- 新增 RDEPUBSafeArea：参考 GKNavigationBarSwift 的 keyWindow 三级查找与临时 window 兜底
- 移除 reflowableContentInsets 中 top/bottom 40 的固定默认值，改按设备真实安全区生成
- 各取安全区处的 ?? .zero 兜底改为回退到 key window 值
- DEBUG 构建下为文本可读区域绘制红色边框，方便查看排版范围

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Sources/RDReaderView/EPUBUI/Settings/RDEPUBReaderConfiguration.swift

@@ -109,7 +109,7 @@ public struct RDEPUBReaderConfiguration: Equatable {
         showsTableOfContents: Bool = true,
         allowsHighlights: Bool = true,
         showsSettingsPanel: Bool = true,
-        reflowableContentInsets: UIEdgeInsets = UIEdgeInsets(top: 40, left: 16, bottom: 40, right: 16),
+        reflowableContentInsets: UIEdgeInsets = RDEPUBSafeArea.defaultReflowableContentInsets(),
         fixedContentInset: UIEdgeInsets = .zero,
         theme: RDEPUBReaderTheme = .light,
         darkImageAdjustmentEnabled: Bool = true,
@@ -172,11 +172,13 @@ extension RDEPUBReaderConfiguration {
     func makePreferences(safeAreaInsets: UIEdgeInsets = .zero) -> RDEPUBPreferences {
         // Use the larger of reflowableContentInsets and safeAreaInsets for each edge
         // to prevent content from being hidden under Dynamic Island / home indicator.
+        // Falls back to the key-window safe area when the caller has no laid-out view.
+        let resolvedSafeAreaInsets = RDEPUBSafeArea.resolve(safeAreaInsets)
         let safeInsets = UIEdgeInsets(
-            top: max(reflowableContentInsets.top, safeAreaInsets.top),
-            left: max(reflowableContentInsets.left, safeAreaInsets.left),
-            bottom: max(reflowableContentInsets.bottom, safeAreaInsets.bottom),
-            right: max(reflowableContentInsets.right, safeAreaInsets.right)
+            top: max(reflowableContentInsets.top, resolvedSafeAreaInsets.top),
+            left: max(reflowableContentInsets.left, resolvedSafeAreaInsets.left),
+            bottom: max(reflowableContentInsets.bottom, resolvedSafeAreaInsets.bottom),
+            right: max(reflowableContentInsets.right, resolvedSafeAreaInsets.right)
         )
         return RDEPUBPreferences(
             fontSize: fontSize,