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/utils/file_utils_test.go

@@ -0,0 +1,59 @@
+package utils
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestIsValidFolder(t *testing.T) {
+	tests := []struct {
+		name     string
+		setup    func(t *testing.T) string
+		expected bool
+	}{
+		{
+			name: "valid directory",
+			setup: func(t *testing.T) string {
+				return t.TempDir()
+			},
+			expected: true,
+		},
+		{
+			name: "file not directory",
+			setup: func(t *testing.T) string {
+				dir := t.TempDir()
+				path := filepath.Join(dir, "file.txt")
+				if err := os.WriteFile(path, []byte("content"), 0644); err != nil {
+					t.Fatal(err)
+				}
+				return path
+			},
+			expected: false,
+		},
+		{
+			name: "nonexistent path",
+			setup: func(t *testing.T) string {
+				return "/nonexistent/path/that/does/not/exist"
+			},
+			expected: false,
+		},
+		{
+			name: "empty string",
+			setup: func(t *testing.T) string {
+				return ""
+			},
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		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)
+			}
+		})
+	}
+}