feat: support custom font files for song titles (Closes #4)

- Improve PdfFontMetrics: use canonical path for cache key, validate
  font file existence, use absolute paths for BaseFont.createFont
- Add font file path resolution in SongbookPipeline (relative to
  project directory)
- Add font file existence validation in Validator.validateConfig
- Add end-to-end tests: custom font loading, umlaut rendering,
  cache deduplication, missing file error
- Document custom font file usage in example songbook.yaml

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

app/src/main/kotlin/de/pfadfinder/songbook/app/SongbookPipeline.kt

@@ -28,9 +28,12 @@ class SongbookPipeline(private val projectDir: File) {
             return BuildResult(false, errors = listOf(ValidationError(configFile.name, null, "songbook.yaml not found")))
         }
         logger.info { "Parsing config: ${configFile.absolutePath}" }
-        val config = ConfigParser.parse(configFile)
+        val rawConfig = ConfigParser.parse(configFile)
 
-        // Validate config
+        // Resolve font file paths relative to the project directory
+        val config = resolveFontPaths(rawConfig)
+
+        // Validate config (including font file existence)
         val configErrors = Validator.validateConfig(config)
         if (configErrors.isNotEmpty()) {
             return BuildResult(false, errors = configErrors)
@@ -149,13 +152,39 @@ class SongbookPipeline(private val projectDir: File) {
         )
     }
 
+    /**
+     * Resolves font file paths relative to the project directory.
+     * If a FontSpec has a `file` property, it is resolved against projectDir
+     * to produce an absolute path.
+     */
+    private fun resolveFontPaths(config: BookConfig): BookConfig {
+        fun FontSpec.resolveFile(): FontSpec {
+            val fontFile = this.file ?: return this
+            val fontFileObj = File(fontFile)
+            // Only resolve relative paths; absolute paths are left as-is
+            if (fontFileObj.isAbsolute) return this
+            val resolved = File(projectDir, fontFile)
+            return this.copy(file = resolved.absolutePath)
+        }
+
+        val resolvedFonts = config.fonts.copy(
+            lyrics = config.fonts.lyrics.resolveFile(),
+            chords = config.fonts.chords.resolveFile(),
+            title = config.fonts.title.resolveFile(),
+            metadata = config.fonts.metadata.resolveFile(),
+            toc = config.fonts.toc.resolveFile()
+        )
+        return config.copy(fonts = resolvedFonts)
+    }
+
     fun validate(): List<ValidationError> {
         val configFile = File(projectDir, "songbook.yaml")
         if (!configFile.exists()) {
             return listOf(ValidationError(configFile.name, null, "songbook.yaml not found"))
         }
 
-        val config = ConfigParser.parse(configFile)
+        val rawConfig = ConfigParser.parse(configFile)
+        val config = resolveFontPaths(rawConfig)
         val errors = mutableListOf<ValidationError>()
         errors.addAll(Validator.validateConfig(config))