feat: add support for foreword/preface pages (Closes #1)

Add ForewordConfig to BookConfig, Foreword model type, ForewordParser for
text files (quote/paragraphs/signatures), ForewordPage in PageContent,
pipeline integration to insert foreword after TOC, and PDF rendering with
styled quote, horizontal rule separator, word-wrapped paragraphs, and
right-aligned signatures.

Also adds Gradle wrapper and adjusts build toolchain for JDK 25 compat.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

model/src/main/kotlin/de/pfadfinder/songbook/model/BookConfig.kt

@@ -7,7 +7,12 @@ data class BookConfig(
     val layout: LayoutConfig = LayoutConfig(),
     val images: ImagesConfig = ImagesConfig(),
     val referenceBooks: List<ReferenceBook> = emptyList(),
-    val output: OutputConfig = OutputConfig()
+    val output: OutputConfig = OutputConfig(),
+    val foreword: ForewordConfig? = null
+)
+
+data class ForewordConfig(
+    val file: String = "./foreword.txt"
 )
 
 data class BookMeta(

model/src/main/kotlin/de/pfadfinder/songbook/model/Foreword.kt

@@ -0,0 +1,7 @@
+package de.pfadfinder.songbook.model
+
+data class Foreword(
+    val quote: String? = null,
+    val paragraphs: List<String> = emptyList(),
+    val signatures: List<String> = emptyList()
+)

model/src/main/kotlin/de/pfadfinder/songbook/model/Layout.kt

@@ -10,6 +10,7 @@ sealed class PageContent {
     data class SongPage(val song: Song, val pageIndex: Int) : PageContent()  // pageIndex 0 or 1 for 2-page songs
     data class FillerImage(val imagePath: String) : PageContent()
     data object BlankPage : PageContent()
+    data class ForewordPage(val foreword: Foreword, val pageIndex: Int) : PageContent()  // pageIndex 0 or 1 for multi-page forewords
 }
 
 data class LayoutResult(