Allow saving generated ControlNet previews

[jtreminio]

CleanShot.2026-05-02.at.13.05.55.mp4

[jtreminio]

Per discord conversation, should save directly to inputs/controlnet and not inputs/_comfyN/controlnet. The existing design is clear now, and this PR is much smaller.

[jtreminio]

With the recent refactor, now getting an error. Setting this back to draft while I hunt down the issue:

21:49:35.490 [Error] Error in websocket handler: System.AggregateException: One or more errors occurred. (Invalid value for parameter ControlNet Image Input)
 ---> System.Exception: Invalid value for parameter ControlNet Image Input
 ---> System.IO.DirectoryNotFoundException: Could not find a part of the path '/workspace/Output/local/inputs/controlnet/controlnet-preview-2026-05-09_21-49-10-336.mp4'.
   at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirError)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Boolean failForSymlink, Boolean& wasSymlink, Func`4 createOpenException)
   at System.IO.File.ReadAllBytes(String path)
   at SwarmUI.Text2Image.T2IParamTypes.FilePathToDataString(Session session, String filePath, String errorContext) in /workspace/src/Text2Image/T2IParamTypes.cs:line 1143
   at SwarmUI.Text2Image.T2IParamTypes.ValidateParam(T2IParamType type, String val, Session session) in /workspace/src/Text2Image/T2IParamTypes.cs:line 1075
   at SwarmUI.Text2Image.T2IParamTypes.ApplyParameter(String paramTypeName, String value, T2IParamInput data, Int32 sectionId) in /workspace/src/Text2Image/T2IParamTypes.cs:line 1168
   --- End of inner exception stack trace ---
   at SwarmUI.Text2Image.T2IParamTypes.ApplyParameter(String paramTypeName, String value, T2IParamInput data, Int32 sectionId) in /workspace/src/Text2Image/T2IParamTypes.cs:line 1177
   at SwarmUI.WebAPI.T2IAPI.RequestToParams(Session session, JObject rawInput, Boolean applyPresets) in /workspace/src/WebAPI/T2IAPI.cs:line 213
   at SwarmUI.WebAPI.T2IAPI.GenT2I_Internal(Session session, ValueTuple`4 input, Action`1 output, Boolean isWS) in /workspace/src/WebAPI/T2IAPI.cs:line 268
   --- End of inner exception stack trace ---