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>

cli/src/main/kotlin/de/pfadfinder/songbook/cli/BuildCommand.kt

@@ -18,7 +18,7 @@ class BuildCommand : CliktCommand(name = "build") {
         echo("Building songbook from: ${dir.path}")
 
         val pipeline = SongbookPipeline(dir)
-        val result = pipeline.build()
+        val result = pipeline.build(onProgress = { msg -> echo(msg) })
 
         if (result.success) {
             echo("Build successful!")