fix(converter): preserve OriginalName through conversion and fix race

- Fix critical bug: convertPageFile and splitAndConvert now copy
  OriginalName to the output PageFile so --keep-filenames works
  end-to-end on actually-converted pages.
- Fix concurrency race: ExtractChapter deduplicates OriginalName when
  source archive has same base filename in different subdirectories.
- Fix collision fallback: resolvePageName loops until an unused
  indexed name is found instead of single-shot assignment.
- Fix gofmt indentation in optimize_command.go.

internal/cbz/cbz_loader.go

@@ -168,6 +168,18 @@ func ExtractChapter(ctx context.Context, filePath string, keepFilenames bool) (*
 		TempDir:  tempDir,
 	}
 
+	// usedOriginalNames tracks every OriginalName handed out in this chapter
+	// so it stays unique across pages, even when the source archive contains
+	// the same base filename in different subdirectories (e.g. vol1/page.png
+	// and vol2/page.png — legal in zip). Uniqueness at the extraction
+	// boundary prevents a downstream race in pkg/converter/webp, where two
+	// pages with the same stem would write the same intermediate file.
+	// Allocated lazily so the keepFilenames=false path stays allocation-free.
+	var usedOriginalNames map[string]struct{}
+	if keepFilenames {
+		usedOriginalNames = make(map[string]struct{})
+	}
+
 	// For CBZ files, read metadata from zip comment
 	pathLower := strings.ToLower(filepath.Ext(filePath))
 	if pathLower == ".cbz" {
@@ -286,7 +298,7 @@ func ExtractChapter(ctx context.Context, filePath string, keepFilenames bool) (*
 			FilePath:  outputPath,
 		}
 		if keepFilenames {
-			page.OriginalName = filepath.Base(path)
+			page.OriginalName = allocateUniqueBaseName(filepath.Base(path), pageIndex, usedOriginalNames)
 		}
 		chapter.Pages = append(chapter.Pages, page)
 
@@ -328,6 +340,33 @@ func isJunkFile(path string) bool {
 	return false
 }
 
+// allocateUniqueBaseName returns baseName when it is not already taken by an
+// earlier page in this chapter, or a collision-resolved variant otherwise.
+//
+// On collision the resolved name matches the existing fallback style used by
+// cbz_creator's resolvePageName: stem + "_%04d" + extension. The starting
+// suffix is pageIndex so the resolved name stays in the same neighborhood as
+// the page's archive position. If that candidate is itself already taken
+// (astronomically rare: the source archive would need both the original name
+// and a matching indexed name), the suffix is incremented until a free name
+// is found. The chosen name is recorded in usedNames so subsequent calls
+// cannot pick it again.
+func allocateUniqueBaseName(baseName string, pageIndex uint16, usedNames map[string]struct{}) string {
+	if _, taken := usedNames[baseName]; !taken {
+		usedNames[baseName] = struct{}{}
+		return baseName
+	}
+	ext := filepath.Ext(baseName)
+	stem := strings.TrimSuffix(baseName, ext)
+	for suffix := int(pageIndex); ; suffix++ {
+		candidate := fmt.Sprintf("%s_%04d%s", stem, suffix, ext)
+		if _, taken := usedNames[candidate]; !taken {
+			usedNames[candidate] = struct{}{}
+			return candidate
+		}
+	}
+}
+
 // LoadChapter extracts the chapter from a CBZ/CBR file to disk.
 // It delegates to ExtractChapter with keepFilenames=false and always
 // extracts all pages. Use IsAlreadyConverted for a fast conversion status