feat: support inline images within song pages (Closes #2)

Add {image: path} directive to embed images at any position within a song's
sections. SongLine gains an optional imagePath field; when set, the line
represents an inline image rather than chord/lyric content. The renderer
scales and centers images within the content width. MeasurementEngine
reserves 40mm height per inline image for layout calculations.

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

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

@@ -23,7 +23,10 @@ enum class SectionType {
     VERSE, CHORUS, BRIDGE, REPEAT
 }
 
-data class SongLine(val segments: List<LineSegment>)
+data class SongLine(
+    val segments: List<LineSegment> = emptyList(),
+    val imagePath: String? = null  // when non-null, this "line" is an inline image (segments ignored)
+)
 
 data class LineSegment(
     val chord: String? = null,  // null = no chord above this segment