User-configured preview value isn't included in JSON object sent for GPU job mutation

The following code snippet sets the preview value to `null` for both detector dimensions, regardless of what the preview value was set by the user: https://github.com/DiamondLightSource/TomoHub/blob/ea9a63c1443cda6b132ead9a4f2b32b8d316e210/frontend/src/hooks/useHTTOMOConfig.ts#L114-L118

This essentially means that for all job submission forms that use this hook to get the loader config (which is at least all high-throughput jobs submitted by `SubmissionFormGPURun`), the user-configured preview value is ignored, and preview configuration that selects the full data is sent to httomo.

Start at frontend/src/hooks/useHTTOMOConfig.ts lines 114-118 and trace how SubmissionFormGPURun consumes the loader config. Verify the JSON object sent for GPU job mutation retains the user-configured preview value for both detector dimensions instead of always sending null, then confirm full-data preview configuration is no longer sent when a user selects another value.

Written by the indexing model from the issue text.