A Radix <Select> renders a blank trigger when its `value` matches no
<SelectItem>. The Settings model pickers built their options solely from
each provider's curated `models` list, so a model added via config that
isn't in that list (e.g. anthropic/claude-opus-4.7 on nous) selected
nothing and showed an empty selector.
Union the active value into the options via a small `withActive` helper,
applied to the main, auxiliary, MoA reference, and MoA aggregator model
selects so the configured model always stays visible and selectable.
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