v1.12.2: live tok/s + ctx display next to status indicator

ChatThroughput renders inline beside StatusDot while streaming or
tool_running. Subscribes to existing usage frames via sessionEvents.
Hides when status drops to idle/error or data is older than 10s.

Addresses the 2026-05-21 spike's UX gap where slow streams looked
identical to dead streams — now there's a live token velocity readout
that immediately distinguishes the two.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/api/types.ts

@@ -332,6 +332,17 @@ export type WsFrame =
       // to the client without a refetch.
       metadata?: MessageMetadata | null;
     }
+  // v1.12.2: live throughput frame, published mid-stream every ~500ms with
+  // the latest token + ctx counts so ChatThroughput can render tok/s and
+  // ctx_used while the model is still generating.
+  | {
+      type: 'usage';
+      message_id: string;
+      chat_id?: string;
+      completion_tokens: number | null;
+      ctx_used: number | null;
+      ctx_max: number | null;
+    }
   | { type: 'messages_deleted'; message_ids: string[]; chat_id?: string }
   | { type: 'chat_renamed'; chat_id: string; name: string }
   // v1.11: published by services/compaction.ts after the new anchored