AssetManager 0.5.5.53

MeshCentral device icon integration

Separates the MeshCentral group type (mtype) from the per-device node.icon / agent platform type.
Stores the MeshCentral device icon in the new read-only mesh_icon asset field.
Uses mesh_icon for category assignment when new assets are created from MeshCentral; existing category mapping values 1-8 remain compatible.
Repairs only the known historical system mapping that combined mtype and icon; user-created field mappings are left unchanged.
Shows MeshCentral device icon previews in the category mapping UI.
Adds a visual device column to the asset table directly after the row number: an uploaded asset image is preferred, otherwise the corresponding MeshCentral device icon is shown.
Redistributes the MeshCentral device icon assets used by this feature under the upstream Apache License 2.0; see THIRD_PARTY_NOTICES.md.

Existing assets are not automatically recategorized by this change.
Run a MeshCentral synchronization to populate mesh_icon for existing linked assets.
The existing mesh_mtype field remains available and now explicitly represents the MeshCentral group type only.