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/ReaderController/RDEPUBReaderEnvironment.swift

@@ -47,7 +47,7 @@ final class RDEPUBReaderEnvironment {
         return RDEPUBNavigatorLayoutContext(
             containerSize: resolvedSize,
             pagesPerScreen: readerView?.pagesPerScreen ?? 1,
-            safeAreaInsets: controller?.view.safeAreaInsets ?? .zero,
+            safeAreaInsets: RDEPUBSafeArea.resolve(controller?.view.safeAreaInsets),
             userInterfaceIdiom: controller?.traitCollection.userInterfaceIdiom ?? .phone,
             reflowableContentInsets: configuration.reflowableContentInsets
         )
@@ -68,7 +68,7 @@ final class RDEPUBReaderEnvironment {
         configuration: RDEPUBReaderConfiguration,
         pageSize: CGSize
     ) -> RDEPUBTextLayoutConfig {
-        let safeAreaInsets = controller?.view.safeAreaInsets ?? .zero
+        let safeAreaInsets = RDEPUBSafeArea.resolve(controller?.view.safeAreaInsets)
         return RDEPUBTextLayoutConfig(
             frameWidth: max(pageSize.width, 1),
             frameHeight: max(pageSize.height, 1),