fix: normalize raster images before img2pdf retry

img2pdf refuses images with alpha channels or more than 8 bits per
channel (common for screenshots). Normalize such images to opaque
8-bit with ImageMagick and retry img2pdf before falling back to a
full ImageMagick PDF conversion.

src/main.rs

@@ -842,7 +842,8 @@ fn path_hash(path: &Path) -> u64 {
 /// Print the currently active document to the system printer via CUPS (lp).
 ///
 /// - **PDF** → sent directly to CUPS via `lp`
-/// - **Raster images** → converted to PDF via `img2pdf` (or `convert` as fallback)
+/// - **Raster images** → converted to PDF via `img2pdf` (normalized to
+///   opaque 8-bit when needed, or `convert` as fallback)
 /// - **SVG** → converted to PDF via `pdftocairo` → `img2pdf`
 fn print_document_sync(path: &Path, kind: DocumentKind) -> Result<(), String> {
     let pdf_path = match kind {
@@ -861,10 +862,45 @@ fn convert_raster_to_pdf(path: &Path) -> Result<PathBuf, String> {
         return Ok(result);
     }
 
+    // img2pdf refuses alpha channels and >8 bit channels: normalize to
+    // opaque 8-bit and retry before falling back to ImageMagick.
+    if let Ok(normalized) = normalize_raster(path) {
+        let result = convert_with_cmd(
+            "img2pdf",
+            vec![normalized.to_string_lossy().into_owned()],
+            path,
+        );
+        let _ = std::fs::remove_file(&normalized);
+        if let Ok(result) = result {
+            return Ok(result);
+        }
+    }
+
     // Fallback: ImageMagick convert
     convert_with_cmd("convert", vec![path.to_string_lossy().into_owned()], path)
 }
 
+fn normalize_raster(path: &Path) -> Result<PathBuf, String> {
+    let extension = path.extension().and_then(OsStr::to_str).unwrap_or("png");
+    let output_path = temp_prefix(path, "normalized").with_extension(extension);
+
+    let status = Command::new("convert")
+        .arg(path)
+        .args(["-depth", "8", "-alpha", "off"])
+        .arg(&output_path)
+        .status()
+        .map_err(|e| format!("Failed to run convert: {e}"))?;
+    if !status.success() {
+        return Err("convert failed".to_string());
+    }
+
+    if output_path.exists() {
+        Ok(output_path)
+    } else {
+        Err("convert did not produce output".to_string())
+    }
+}
+
 fn convert_svg_to_pdf(path: &Path) -> Result<PathBuf, String> {
     // Render SVG to PNG with pdftocairo, then convert PNG to PDF
     let prefix = temp_prefix(path, "svg");