Fix part validation blocking unit creation

Creating a unit from a part with ., :, or + in its number used to fail with a raw validation blob. Validation now matches what part creation allows, and errors are readable.

• Fixed unit creation rejecting valid parts whose numbers contain ., :, or +
• Improved error messages in modals to show a clear one-line reason instead of a raw JSON payload
• Added a Copy action on error toasts so you can share the exact message with support