feat: decode activity SSE events and expose tracker total

src/lib/inflight-tracker.ts

@@ -15,6 +15,7 @@ export interface ModelState {
 export type FeedEvent =
   | { type: "inflight"; operation: "snapshot" | "add"; requests: InflightRequest[] }
   | { type: "inflight"; operation: "remove"; id: string }
+  | { type: "activity"; id: number }
   | { type: "modelStatus"; models: ModelState[] };
 
 export type ModelRuntimeState = "stopped" | "loading" | "ready";
@@ -44,6 +45,10 @@ export class InflightTracker {
     return n;
   }
 
+  total(): number {
+    return this.requests.size;
+  }
+
   state(modelId: string): ModelRuntimeState | undefined {
     return this.states.get(modelId);
   }