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/optimize_integration_test.go

@@ -213,7 +213,7 @@ func TestOptimizeIntegration(t *testing.T) {
 			}
 
 			// Clean up output file
-			os.Remove(expectedOutput)
+			_ = os.Remove(expectedOutput)
 		})
 	}
 }
@@ -394,7 +394,7 @@ func TestOptimizeIntegration_Timeout(t *testing.T) {
 		Quality:          85,
 		Override:         false,
 		Split:            false,
-		Timeout:          10 * time.Millisecond, // Very short timeout to force timeout
+		Timeout:          1 * time.Nanosecond, // Extremely short timeout to force timeout
 	}
 
 	err = Optimize(options)