feat: add intro page, rotated TOC headers, progress indicator, fix TOC padding (Closes #33)

- Add intro/title page before TOC (configurable via intro.enabled in songbook.yaml)
- Rotate reference book column headers 90° in TOC to prevent text truncation
- Add progress callback to SongbookPipeline.build() with CLI and GUI integration
- Make GUI song loading async with spinner indicator
- Fix TOC page padding: use actual rendered page count instead of blindly adding tocPages-1 blank pages
- Pre-render TOC to measure actual pages needed (TocRenderer.measurePages)
- Account for intro pages in pagination offset and page numbering

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

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

@@ -9,7 +9,12 @@ data class BookConfig(
     val referenceBooks: List<ReferenceBook> = emptyList(),
     val output: OutputConfig = OutputConfig(),
     val foreword: ForewordConfig? = null,
-    val toc: TocConfig = TocConfig()
+    val toc: TocConfig = TocConfig(),
+    val intro: IntroConfig? = null
+)
+
+data class IntroConfig(
+    val enabled: Boolean = true
 )
 
 data class TocConfig(

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

@@ -14,6 +14,7 @@ sealed class PageContent {
 }
 
 data class LayoutResult(
+    val introPages: Int = 0,
     val tocPages: Int,
     val pages: List<PageContent>,
     val tocEntries: List<TocEntry>