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>

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 = """