refactor!: disk-first zero-copy architecture with file-to-file cwebp conversion

BREAKING CHANGE: Complete refactoring of the conversion pipeline.

- Replace in-memory Page/PageContainer with disk-based PageFile struct
- Extract archives to temp directory instead of loading into memory
- Use cwebp InputFile/OutputFile for zero-copy file-to-file conversion
- Use cwebp -crop for splitting (no Go-side image decode needed)
- Check if already converted before extracting (fast pre-check)
- Remove oliamb/cutter dependency (replaced by cwebp -crop)
- Remove page_container.go (no longer needed)
- Add comprehensive tests with improved coverage

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

internal/manga/manga_test.go

@@ -0,0 +1,102 @@
+package manga
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestChapter_SetConverted(t *testing.T) {
+	chapter := &Chapter{}
+
+	if chapter.IsConverted {
+		t.Error("New chapter should not be converted")
+	}
+	if !chapter.ConvertedTime.IsZero() {
+		t.Error("New chapter should have zero ConvertedTime")
+	}
+
+	before := time.Now()
+	chapter.SetConverted()
+	after := time.Now()
+
+	if !chapter.IsConverted {
+		t.Error("Chapter should be converted after SetConverted()")
+	}
+	if chapter.ConvertedTime.Before(before) || chapter.ConvertedTime.After(after) {
+		t.Error("ConvertedTime should be between before and after")
+	}
+}
+
+func TestChapter_Cleanup(t *testing.T) {
+	// Create a temp dir with some files
+	tmpDir := t.TempDir()
+	chapterDir := filepath.Join(tmpDir, "chapter-cleanup-test")
+	if err := os.MkdirAll(filepath.Join(chapterDir, "input"), 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(chapterDir, "input", "page.jpg"), []byte("test"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	chapter := &Chapter{TempDir: chapterDir}
+
+	err := chapter.Cleanup()
+	if err != nil {
+		t.Fatalf("Cleanup failed: %v", err)
+	}
+
+	if chapter.TempDir != "" {
+		t.Error("TempDir should be empty after cleanup")
+	}
+
+	if _, err := os.Stat(chapterDir); !os.IsNotExist(err) {
+		t.Error("Directory should not exist after cleanup")
+	}
+}
+
+func TestChapter_Cleanup_EmptyTempDir(t *testing.T) {
+	chapter := &Chapter{TempDir: ""}
+
+	err := chapter.Cleanup()
+	if err != nil {
+		t.Errorf("Cleanup with empty TempDir should not error, got: %v", err)
+	}
+}
+
+func TestChapter_Cleanup_NonexistentDir(t *testing.T) {
+	chapter := &Chapter{TempDir: "/nonexistent/path/that/does/not/exist"}
+
+	// os.RemoveAll on a nonexistent path returns nil
+	err := chapter.Cleanup()
+	if err != nil {
+		t.Errorf("Cleanup of nonexistent dir should not error, got: %v", err)
+	}
+}
+
+func TestPageFile_Struct(t *testing.T) {
+	page := &PageFile{
+		Index:          5,
+		Extension:      ".webp",
+		FilePath:       "/tmp/test/0005.webp",
+		IsSplitted:     true,
+		SplitPartIndex: 2,
+	}
+
+	if page.Index != 5 {
+		t.Errorf("Expected Index 5, got %d", page.Index)
+	}
+	if page.Extension != ".webp" {
+		t.Errorf("Expected Extension .webp, got %s", page.Extension)
+	}
+	if page.FilePath != "/tmp/test/0005.webp" {
+		t.Errorf("Expected FilePath /tmp/test/0005.webp, got %s", page.FilePath)
+	}
+	if !page.IsSplitted {
+		t.Error("Expected IsSplitted true")
+	}
+	if page.SplitPartIndex != 2 {
+		t.Errorf("Expected SplitPartIndex 2, got %d", page.SplitPartIndex)
+	}
+}