The Add Computer, Add Machine, and Add Printer forms offered the entire model catalog and vendor list regardless of what was being added, so a PC could be pointed at a printer model and every vendor showed up whether or not it made that kind of thing. Scope each form to its own asset category so the selectors only present sensible choices.
Add a ?category= filter to GET /api/models that joins modeltypes and matches on category (Equipment, PC, Network, Printer), and include the model's category in the list response so the forms can distinguish types client-side. Untyped models are excluded, since a null category cannot satisfy the filter.
Each subtype form now loads only its category: PCForm requests category PC (servers, which are Network, stay out); MachineForm requests category Equipment, with the main Model dropdown hiding Controller-type models and the Controller dropdown showing only those; PrinterForm requests category Printer, which also fixes its Model list that previously showed the whole catalog.
Vendor dropdowns are derived from the loaded models: a form only offers vendors that actually have a model in its category. Each dropdown keeps the record's own saved vendor even if it falls outside that set, so editing an existing asset never drops its vendor.
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