fix: address copilot and coderabbit review feedback

- Use named returns for IsAlreadyConverted and WriteChapterToCBZ so
  errs.Capture defers propagate close errors to the caller.
- Add context.Context to IsAlreadyConverted and ExtractChapter for
  cancellation/timeout support; thread it from Optimize.
- Filter non-image files (__MACOSX, Thumbs.db, .DS_Store, etc.) and
  only extract supported image extensions in ExtractChapter.
- Validate TempDir is non-empty in ConvertChapter before creating output dir.
- Separate fatal conversion errors from PageIgnoredError in the WebP
  converter result aggregation so real failures aren't masked.
- Fix error message "failed to load chapter" → "failed to extract chapter".
- Use %w instead of %v in fmt.Errorf for proper error wrapping.
- Fix LoadChapter doc comment to match its actual behavior.
- Extract shared parseConvertedComment/parseConvertedCommentTime helpers
  to avoid duplicated zip-comment parsing logic.
- Convert tests to use testify assertions (assert/require) per guidelines.

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

internal/utils/file_utils_test.go

@@ -4,6 +4,8 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestIsValidFolder(t *testing.T) {
@@ -51,9 +53,7 @@ func TestIsValidFolder(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			path := tt.setup(t)
 			result := IsValidFolder(path)
-			if result != tt.expected {
-				t.Errorf("IsValidFolder(%q) = %v, want %v", path, result, tt.expected)
-			}
+			assert.Equal(t, tt.expected, result)
 		})
 	}
 }