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>

buildSrc/src/main/kotlin/songbook-conventions.gradle.kts

@@ -2,8 +2,15 @@ plugins {
     kotlin("jvm")
 }
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_21
+    targetCompatibility = JavaVersion.VERSION_21
+}
+
 kotlin {
-    jvmToolchain(21)
+    compilerOptions {
+        jvmTarget.set(org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_21)
+    }
 }
 
 tasks.withType<Test> {