fix(byok): decouple chat model picker from config.models

[Gagancreates]

Problem

After BYOK onboarding, Settings → Models showed a stack of separate assistant-model dropdowns — one per model in the provider catalog — and an "Add assistant model" button. Root cause: config.models served double duty for BYOK (signed-out):

1. It seeded the in-chat model picker (chat-input-with-mentions.tsx).
2. It was rendered as a per-entry dropdown stack in Settings.

Onboarding persisted the provider's entire fetched catalog into config.models, so every model became a Settings row. Trimming it to one model would've fixed Settings but removed multi-model choice in chat — the coupling forced a bad tradeoff, and also baked a stale catalog snapshot into config.

Fix — decouple the two, so BYOK matches the signed-in experience

• Onboarding (use-onboarding-state.ts): seed config.models with just the selected default model, not the whole catalog.
• Chat picker (chat-input-with-mentions.tsx): signed-out branch now lists the full live provider catalog via models:list, filtered to providers the user has a key/baseURL for, with the saved default model leading. No longer limited to config.models, so newly released models appear without re-saving.
• Settings (settings-dialog.tsx): ASSISTANT MODEL collapses to a single default-model dropdown (removed the .map() stack + add/remove). Underlying models: string[] data model is unchanged, so save/load/delete logic is untouched; config.models is now effectively [defaultModel].

Migration

Configs written by the old code keep a bloated models array. Opening Settings → Models and saving collapses it to the single selected model. The chat picker ignores config.models entirely now, so old configs already behave correctly there.

Test plan

• Fresh BYOK onboarding (Anthropic key)
• Settings → Models: ASSISTANT MODEL is a single dropdown (no stack / no "Add assistant model")
• Chat model picker: lists the full provider catalog, default model first, switchable
• Signed-in path unchanged (still uses the gateway list)

Checks

• tsc --noEmit clean across the renderer
• npm run lint: only pre-existing errors (none in the changed files)