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

cmd/cbzoptimizer/commands/watch_command.go

@@ -65,6 +65,8 @@ func WatchCommand(_ *cobra.Command, args []string) error {
 
 	split := viper.GetBool("split")
 
+	keepFilenames := viper.GetBool("keep-filenames")
+
 	timeout := viper.GetDuration("timeout")
 
 	backfill := viper.GetBool("backfill")
@@ -79,7 +81,7 @@ func WatchCommand(_ *cobra.Command, args []string) error {
 	if err != nil {
 		return fmt.Errorf("failed to prepare converter: %w", err)
 	}
-	log.Info().Str("path", path).Bool("override", override).Uint8("quality", quality).Str("format", converterType.String()).Bool("split", split).Bool("backfill", backfill).Msg("Watching directory")
+	log.Info().Str("path", path).Bool("override", override).Uint8("quality", quality).Str("format", converterType.String()).Bool("split", split).Bool("keep_filenames", keepFilenames).Bool("backfill", backfill).Msg("Watching directory")
 
 	watcher, err := fsnotify.NewWatcher()
 	if err != nil {
@@ -98,6 +100,7 @@ func WatchCommand(_ *cobra.Command, args []string) error {
 		Quality:          quality,
 		Override:         override,
 		Split:            split,
+		KeepFilenames:    keepFilenames,
 		Timeout:          timeout,
 	})
 	defer queue.Stop()