Move Mux Gateway routing to backend

[ethanndickson]

Summary

First message in a new workspace can bypass mux‑gateway and fail with “API key not found for anthropic.” Retry succeeds after gateway state syncs. Root cause is frontend/localStorage timing; fix by moving routing decision to backend.

Observed

• First message fails with API key not found for anthropic
• Manual retry succeeds via mux-gateway

Root Cause (frontend race)

Gateway routing is determined in the browser via toGatewayModel() + localStorage:

1. gateway-available defaults to false
2. useProvidersConfig() loads async
3. useGateway() writes gateway-available after config loads
4. First send can happen before sync → model stays anthropic:*

Why name generation still works

Name generation runs on the backend (nameGeneration.generate) and selects a model server‑side via aiService.createModel(), which reads providers.jsonc every time. It can choose mux‑gateway variants even if frontend localStorage is stale.

Proposed Fix

Move gateway routing to the backend:

• Backend receives canonical provider:model
• Backend decides mux‑gateway routing from provider config on disk
• Backend rewrites to mux-gateway:provider/model when enabled
• Frontend toggles remain UI + send flags, but routing becomes authoritative server‑side

[ethanndickson]

Enables #1208