Feature: Option to move existing assets when changing input/output directories

## Request

When changing the input or output directory in Desktop Settings, provide an option to automatically move all existing assets from the old directory to the new directory.

## Current Behavior

Currently, users can change the input and output directory paths in the Desktop Settings, but the existing files remain in the old locations. Users must manually move files if they want to consolidate everything in the new location.


## Proposed Behavior

When a user changes the input or output directory, show a dialog or checkbox option asking if they want to move existing files from the old location to the new location.

Source: #temp-desktop-bug-dump, January 8th.

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*Migrated from Linear: [DESK2-116](https://linear.app/comfyorg/issue/DESK2-116/feature-option-to-move-existing-assets-when-changing-inputoutput)*