feat(activity): fan out activity updates to all connected instances

Switch pushActivities and setShowActivity from wsSend (home-only) to
wsSendAll (all instances). Add myActivities cache for re-push on late
connections.

packages/web/src/stores/activityFanout.test.ts

@@ -0,0 +1,85 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+
+// Mock useWebSocket before importing activityStore
+const mockWsSend = vi.fn();
+const mockWsSendAll = vi.fn();
+vi.mock('../hooks/useWebSocket', () => ({
+  wsSend: (...args: unknown[]) => mockWsSend(...args),
+  wsSendAll: (...args: unknown[]) => mockWsSendAll(...args),
+}));
+
+import { useActivityStore } from './activityStore';
+import type { Activity } from '@backspace/shared';
+
+const GAME_ACTIVITY: Activity = {
+  type: 'playing',
+  name: 'Minecraft',
+  timestamps: { start: Date.now() },
+};
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  useActivityStore.getState().reset();
+  vi.useFakeTimers();
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe('activityStore fan-out', () => {
+  it('pushActivities caches myActivities immediately before debounce', () => {
+    useActivityStore.getState().pushActivities([GAME_ACTIVITY]);
+
+    // myActivities should be set immediately (before the 5s debounce)
+    expect(useActivityStore.getState().myActivities).toEqual([GAME_ACTIVITY]);
+
+    // wsSendAll should NOT have been called yet (debounce hasn't fired)
+    expect(mockWsSendAll).not.toHaveBeenCalled();
+  });
+
+  it('pushActivities calls wsSendAll (not wsSend) after debounce', () => {
+    useActivityStore.getState().pushActivities([GAME_ACTIVITY]);
+    vi.advanceTimersByTime(5000);
+
+    expect(mockWsSendAll).toHaveBeenCalledWith({
+      type: 'activity_update',
+      activities: [GAME_ACTIVITY],
+    });
+    expect(mockWsSend).not.toHaveBeenCalled();
+  });
+
+  it('setShowActivity(false) fans out empty activities via wsSendAll and clears myActivities', () => {
+    // First set some activities
+    useActivityStore.getState().pushActivities([GAME_ACTIVITY]);
+    expect(useActivityStore.getState().myActivities).toEqual([GAME_ACTIVITY]);
+
+    // Now disable
+    useActivityStore.getState().setShowActivity(false);
+
+    expect(mockWsSendAll).toHaveBeenCalledWith({
+      type: 'activity_update',
+      activities: [],
+    });
+    expect(useActivityStore.getState().myActivities).toBeNull();
+  });
+
+  it('reset clears myActivities', () => {
+    useActivityStore.getState().pushActivities([GAME_ACTIVITY]);
+    expect(useActivityStore.getState().myActivities).toEqual([GAME_ACTIVITY]);
+
+    useActivityStore.getState().reset();
+    expect(useActivityStore.getState().myActivities).toBeNull();
+  });
+
+  it('pushActivities does nothing when showActivity is false', () => {
+    useActivityStore.getState().setShowActivity(false);
+    vi.clearAllMocks();
+
+    useActivityStore.getState().pushActivities([GAME_ACTIVITY]);
+    expect(useActivityStore.getState().myActivities).toBeNull();
+
+    vi.advanceTimersByTime(5000);
+    expect(mockWsSendAll).not.toHaveBeenCalled();
+  });
+});

packages/web/src/stores/activityStore.ts

@@ -1,12 +1,13 @@
 import { create } from 'zustand';
 import type { Activity } from '@backspace/shared';
-import { wsSend } from '../hooks/useWebSocket';
+import { wsSendAll } from '../hooks/useWebSocket';
 
 let pushTimer: ReturnType<typeof setTimeout> | null = null;
 
 interface ActivityState {
   userActivities: Map<string, Activity[]>;
   showActivity: boolean;
+  myActivities: Activity[] | null;
 
   setUserActivities: (userId: string, activities: Activity[]) => void;
   clearUserActivities: (userId: string) => void;
@@ -19,6 +20,7 @@ interface ActivityState {
 export const useActivityStore = create<ActivityState>((set, get) => ({
   userActivities: new Map(),
   showActivity: true,
+  myActivities: null,
 
   setUserActivities: (userId, activities) => {
     set((state) => {
@@ -58,21 +60,23 @@ export const useActivityStore = create<ActivityState>((set, get) => ({
     set({ showActivity: show });
     if (!show) {
       if (pushTimer) { clearTimeout(pushTimer); pushTimer = null; }
-      wsSend({ type: 'activity_update', activities: [] });
+      wsSendAll({ type: 'activity_update', activities: [] });
+      set({ myActivities: null });
     }
   },
 
   pushActivities: (activities) => {
     if (!get().showActivity) return;
+    set({ myActivities: activities });
     if (pushTimer) clearTimeout(pushTimer);
     pushTimer = setTimeout(() => {
-      wsSend({ type: 'activity_update', activities });
+      wsSendAll({ type: 'activity_update', activities });
       pushTimer = null;
     }, 5000);
   },
 
   reset: () => {
     if (pushTimer) { clearTimeout(pushTimer); pushTimer = null; }
-    set({ userActivities: new Map(), showActivity: true });
+    set({ userActivities: new Map(), showActivity: true, myActivities: null });
   },
 }));