Fix format flag crash and add comprehensive tests

- Remove NoOptDefVal which caused the format flag to fail with space-separated values
- Add 5 comprehensive unit tests for format flag: space syntax, short form, equals syntax, default value, and case-insensitive
- Update README with detailed format flag documentation and examples
- Format flag now works with all syntaxes: --format webp, -f webp, --format=webp
- Default value (webp) is preserved and shown in help text

Co-authored-by: Belphemur <197810+Belphemur@users.noreply.github.com>

cmd/cbzoptimizer/commands/optimize_command.go

@@ -33,11 +33,10 @@ func init() {
 	command.Flags().BoolP("override", "o", false, "Override the original CBZ/CBR files")
 	command.Flags().BoolP("split", "s", false, "Split long pages into smaller chunks")
 	command.Flags().DurationP("timeout", "t", 0, "Maximum time allowed for converting a single chapter (e.g., 30s, 5m, 1h). 0 means no timeout")
-	command.PersistentFlags().VarP(
+	command.Flags().VarP(
 		formatFlag,
 		"format", "f",
 		fmt.Sprintf("Format to convert the images to: %s", constant.ListAll()))
-	command.PersistentFlags().Lookup("format").NoOptDefVal = constant.DefaultConversion.String()
 
 	AddCommand(command)
 }