feat: highlight the current book's column in the TOC (Closes #6)

Add TocConfig with highlightColumn field to BookConfig. TocRenderer now
applies a light gray background shading to the designated column header
and data cells, making it easy to visually distinguish the current book's
page numbers from reference book columns.

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

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

@@ -8,7 +8,12 @@ data class BookConfig(
     val images: ImagesConfig = ImagesConfig(),
     val referenceBooks: List<ReferenceBook> = emptyList(),
     val output: OutputConfig = OutputConfig(),
-    val foreword: ForewordConfig? = null
+    val foreword: ForewordConfig? = null,
+    val toc: TocConfig = TocConfig()
+)
+
+data class TocConfig(
+    val highlightColumn: String? = null  // abbreviation of the column to highlight (e.g. "CL")
 )
 
 data class ForewordConfig(

parser/src/test/kotlin/de/pfadfinder/songbook/parser/ConfigParserTest.kt

@@ -192,6 +192,28 @@ class ConfigParserTest {
         config.foreword.shouldBeNull()
     }
 
+    @Test
+    fun `parse config with toc highlight column`() {
+        val yaml = """
+            book:
+              title: "Test"
+            toc:
+              highlight_column: "CL"
+        """.trimIndent()
+        val config = ConfigParser.parse(yaml)
+        config.toc.highlightColumn shouldBe "CL"
+    }
+
+    @Test
+    fun `parse config without toc section uses defaults`() {
+        val yaml = """
+            book:
+              title: "Test"
+        """.trimIndent()
+        val config = ConfigParser.parse(yaml)
+        config.toc.highlightColumn.shouldBeNull()
+    }
+
     @Test
     fun `parse config ignores unknown properties`() {
         val yaml = """

renderer-pdf/src/main/kotlin/de/pfadfinder/songbook/renderer/pdf/TocRenderer.kt

@@ -3,11 +3,15 @@ package de.pfadfinder.songbook.renderer.pdf
 import com.lowagie.text.*
 import com.lowagie.text.pdf.*
 import de.pfadfinder.songbook.model.*
+import java.awt.Color
 
 class TocRenderer(
     private val fontMetrics: PdfFontMetrics,
     private val config: BookConfig
 ) {
+    // Light gray background for the highlighted column
+    private val highlightColor = Color(220, 220, 220)
+
     fun render(document: Document, writer: PdfWriter, tocEntries: List<TocEntry>) {
         val tocFont = fontMetrics.getBaseFont(config.fonts.toc)
         val tocBoldFont = fontMetrics.getBaseFontBold(config.fonts.toc)
@@ -35,12 +39,25 @@ class TocRenderer(
         }
         table.setWidths(widths)
 
+        // Determine which column index should be highlighted
+        val highlightAbbrev = config.toc.highlightColumn
+        val highlightColumnIndex: Int? = if (highlightAbbrev != null) {
+            // Check "Seite" (page) column first - the current book's page number column
+            if (highlightAbbrev == "Seite") {
+                1
+            } else {
+                val refIndex = refBooks.indexOfFirst { it.abbreviation == highlightAbbrev }
+                if (refIndex >= 0) 2 + refIndex else null
+            }
+        } else null
+
         // Header row
         val headerFont = Font(tocBoldFont, fontSize, Font.BOLD)
-        table.addCell(headerCell("Titel", headerFont))
+        table.addCell(headerCell("Titel", headerFont, isHighlighted = false))
-        table.addCell(headerCell("Seite", headerFont))
+        table.addCell(headerCell("Seite", headerFont, isHighlighted = highlightColumnIndex == 1))
-        for (book in refBooks) {
+        for ((i, book) in refBooks.withIndex()) {
-            table.addCell(headerCell(book.abbreviation, headerFont))
+            val isHighlighted = highlightColumnIndex == 2 + i
+            table.addCell(headerCell(book.abbreviation, headerFont, isHighlighted = isHighlighted))
         }
         table.headerRows = 1
 
@@ -49,31 +66,43 @@ class TocRenderer(
         val aliasFont = Font(tocFont, fontSize, Font.ITALIC)
         for (entry in tocEntries.sortedBy { it.title.lowercase() }) {
             val font = if (entry.isAlias) aliasFont else entryFont
-            table.addCell(entryCell(entry.title, font))
+            table.addCell(entryCell(entry.title, font, isHighlighted = false))
-            table.addCell(entryCell(entry.pageNumber.toString(), entryFont, Element.ALIGN_RIGHT))
+            table.addCell(entryCell(entry.pageNumber.toString(), entryFont, Element.ALIGN_RIGHT, isHighlighted = highlightColumnIndex == 1))
-            for (book in refBooks) {
+            for ((i, book) in refBooks.withIndex()) {
                 val ref = entry.references[book.abbreviation]
-                table.addCell(entryCell(ref?.toString() ?: "", entryFont, Element.ALIGN_RIGHT))
+                val isHighlighted = highlightColumnIndex == 2 + i
+                table.addCell(entryCell(ref?.toString() ?: "", entryFont, Element.ALIGN_RIGHT, isHighlighted = isHighlighted))
             }
         }
 
         document.add(table)
     }
 
-    private fun headerCell(text: String, font: Font): PdfPCell {
+    private fun headerCell(text: String, font: Font, isHighlighted: Boolean): PdfPCell {
         val cell = PdfPCell(Phrase(text, font))
         cell.borderWidth = 0f
         cell.borderWidthBottom = 0.5f
         cell.paddingBottom = 4f
+        if (isHighlighted) {
+            cell.backgroundColor = highlightColor
+        }
         return cell
     }
 
-    private fun entryCell(text: String, font: Font, alignment: Int = Element.ALIGN_LEFT): PdfPCell {
+    private fun entryCell(
+        text: String,
+        font: Font,
+        alignment: Int = Element.ALIGN_LEFT,
+        isHighlighted: Boolean = false
+    ): PdfPCell {
         val cell = PdfPCell(Phrase(text, font))
         cell.borderWidth = 0f
         cell.horizontalAlignment = alignment
         cell.paddingTop = 1f
         cell.paddingBottom = 1f
+        if (isHighlighted) {
+            cell.backgroundColor = highlightColor
+        }
         return cell
     }
 }