feat(converter): add --keep-filenames flag to preserve original page filenames

Add a new `--keep-filenames` boolean flag (no shorthand, default false) on
both `optimize` and `watch` commands. When enabled, page entries in the
output CBZ preserve their original base filenames instead of being
renumbered to `%04d` sequential indices. Extension is swapped on format
conversion (e.g. `001.png` → `001.webp`), split pages get `-NN` suffixes
on the original stem, and duplicate base names fall back to
`<stem>_<index>.<ext>`. Watch mode supports the flag via the
`keep-filenames` viper config key.

Design: data-driven `OriginalName` field on `manga.PageFile`, set at
extraction in `cbz_loader.ExtractChapter`. The `Converter` interface is
unchanged. Flag off → behavior identical to before.

Closes #216

internal/cbz/cbz_creator.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
+	"strings"
 	"time"
 
 	"github.com/belphemur/CBZOptimizer/v2/internal/manga"
@@ -12,6 +14,49 @@ import (
 	"github.com/rs/zerolog/log"
 )
 
+// resolvePageName picks the final archive entry name for a page.
+//
+// Order of resolution:
+//  1. When page.OriginalName is set (keep-filenames mode), use its stem with
+//     the current page.Extension. Split pages append the -NN suffix so all
+//     parts of the same source still land in the archive.
+//  2. If that final name has already been used in this archive, fall back to
+//     the indexed form (stem + "_%04d" + extension) so the zip stays valid.
+//  3. Otherwise (OriginalName is empty), use the historical %04d / %04d-NN
+//     sequential naming.
+//
+// usedNames tracks every name already chosen for this archive and is mutated
+// in place so the caller can keep a single map across the whole chapter.
+func resolvePageName(page *manga.PageFile, usedNames map[string]struct{}) string {
+	if page.OriginalName != "" {
+		stem := strings.TrimSuffix(page.OriginalName, filepath.Ext(page.OriginalName))
+		var candidate string
+		if page.IsSplitted {
+			candidate = fmt.Sprintf("%s-%02d%s", stem, page.SplitPartIndex, page.Extension)
+		} else {
+			candidate = stem + page.Extension
+		}
+		if _, taken := usedNames[candidate]; !taken {
+			usedNames[candidate] = struct{}{}
+			return candidate
+		}
+		// Collision: fall back to the indexed form so the entry still gets
+		// written, but make it visibly distinct from the preserved one.
+		fallback := fmt.Sprintf("%s_%04d%s", stem, page.Index, page.Extension)
+		usedNames[fallback] = struct{}{}
+		return fallback
+	}
+
+	if page.IsSplitted {
+		name := fmt.Sprintf("%04d-%02d%s", page.Index, page.SplitPartIndex, page.Extension)
+		usedNames[name] = struct{}{}
+		return name
+	}
+	name := fmt.Sprintf("%04d%s", page.Index, page.Extension)
+	usedNames[name] = struct{}{}
+	return name
+}
+
 // WriteChapterToCBZ creates a CBZ file from a Chapter by streaming page files
 // from disk directly into the zip archive. No image data is held in memory.
 func WriteChapterToCBZ(chapter *manga.Chapter, outputFilePath string) (err error) {
@@ -34,14 +79,14 @@ func WriteChapterToCBZ(chapter *manga.Chapter, outputFilePath string) (err error
 	zipWriter := zip.NewWriter(zipFile)
 	defer errs.Capture(&err, zipWriter.Close, "failed to close .cbz writer")
 
-	// Write each page to the archive by streaming from disk
+	// Write each page to the archive by streaming from disk.
+	// Final name resolution: when a page carries an OriginalName (recorded by
+	// ExtractChapter when --keep-filenames is on), preserve its stem and only
+	// swap the extension to the current page.Extension. Duplicates in the
+	// archive fall back to the indexed naming so the output stays a valid zip.
+	usedNames := make(map[string]struct{}, len(chapter.Pages))
 	for _, page := range chapter.Pages {
-		var fileName string
-		if page.IsSplitted {
-			fileName = fmt.Sprintf("%04d-%02d%s", page.Index, page.SplitPartIndex, page.Extension)
-		} else {
-			fileName = fmt.Sprintf("%04d%s", page.Index, page.Extension)
-		}
+		fileName := resolvePageName(page, usedNames)
 
 		log.Debug().
 			Str("output_path", outputFilePath).