Model download/load progress is silently swallowed in browser dev mode (bun run dev) #2983

Problem

A developer using the default bun run dev browser workflow to test native/local chat models gets a blank, phase-less "loading" overlay for the entire download/verify/load duration whenever a model isn't already loaded (indistinguishable from a hang on a large model), but it always eventually resolves and the shipped Tauri app is completely unaffected. The originally reported "clicking download model" trigger is inaccurate — that Settings button is fully mocked in browser mode (downloadViaMock()) and never reaches this buggy code; the real, reproducing trigger is ensure-model-ready, invoked whenever a chat send needs a model that isn't loaded yet.

Root cause

DownloadModel/EnsureModelReady are server-streaming RPCs emitting ModelLoadProgressEvents. The Tauri path forwards each event live via app.emit. The dev-proxy path (packages/dev-tools/src/dev-proxy.ts:957, and the /ensure-model-ready route) instead calls agentStream(...) (grpc-client.ts:310-320), which buffers every event with stream.on('data', ...) and only resolves on stream.on('end', ...) — the HTTP route then awaits the whole array and discards it, responding 204 with no body. The dev-proxy's only SSE bridge relays WatchNodes node-change events, a completely separate stream; nothing ever surfaces ModelLoadProgressEvent to the browser. The UI's phase-label listener is itself gated behind isTauri(), so browser mode shows only a generic overlay with zero phase/progress info for the whole duration.

Suggested fix

Change agentStream's consumption in /ensure-model-ready to relay each ModelLoadProgressEvent over the existing SSE channel as it arrives, instead of buffering the whole stream and discarding it after await.

Found via the 2026-09-25 full-repo code sweep (finding #37 of 84 in CODE_SWEEP_FINDINGS_2026-09-25.md). Severity: medium. Category: broken.