fix(server,app,desktop): route in-app notifications by presence cascade

Replaces the per-client stale/visibility policy with one shared
presence cascade so a backgrounded Firefox tab can no longer steal
in-app notifications from an active Electron desktop window.

Server (`agent-attention-policy.ts`): exports `PRESENCE_THRESHOLD_MS`
(3 min) and `computeNotificationPlan({ allStates, agentId, reason,
nowMs })` returning `{ inAppRecipientIndex, shouldPush }`. Cascade:
focus suppression requires presence; otherwise the most-recent
present client wins (lower-index tiebreak); push fires only when no
client is present, and never on `error`. `broadcastAgentAttention`
calls the policy once per event.

Electron (`daemon-manager.ts`, `idle.ts`): registers
`desktop_get_system_idle_time` returning `powerMonitor
.getSystemIdleTime() * 1000`. Renderer wrapper
`getDesktopSystemIdleTimeMs()` is best-effort: rejected IPC, null,
NaN, Infinity, or negative values return null without throwing.

Hook (`use-client-activity.ts`): Electron-only effect polls every 5s
and updates `lastActivityAtRef` only forward, so OS-wide activity
keeps a backgrounded Electron window present while in-window events
still update immediately.

`ClientHeartbeatMessageSchema` is unchanged; old/new clients and
daemons remain wire-compatible.

Tests: 52 focused tests across policy unit, dispatch, e2e, hook,
desktop wrapper, and permissions. Phase-3 dispatch test +
Phase-5 stale-poll test verified by mutation (breaking the focus
check / the `>` guard makes the exact regression fail).

packages/desktop/src/daemon/daemon-manager.ts

@@ -1,7 +1,7 @@
 import { type ChildProcess } from "node:child_process";
 import { readFileSync } from "node:fs";
 import path from "node:path";
-import { app, ipcMain } from "electron";
+import { app, ipcMain, powerMonitor } from "electron";
 import log from "electron-log/main";
 import { resolvePaseoHome, spawnProcess } from "@getpaseo/server";
 import {
@@ -444,6 +444,10 @@ function resolveCurrentUpdateVersion(): string {
   return resolveDesktopAppVersion();
 }
 
+function getSystemIdleTimeMs(): number {
+  return powerMonitor.getSystemIdleTime() * 1000;
+}
+
 // ---------------------------------------------------------------------------
 // IPC registration
 // ---------------------------------------------------------------------------
@@ -456,6 +460,7 @@ export function createDaemonCommandHandlers(): Record<string, DesktopCommandHand
     restart_desktop_daemon: () => restartDaemon(),
     desktop_daemon_logs: () => getDaemonLogs(),
     desktop_daemon_pairing: () => getDaemonPairing(),
+    desktop_get_system_idle_time: () => getSystemIdleTimeMs(),
     cli_daemon_status: () => getCliDaemonStatus(),
     write_attachment_base64: (args) => writeAttachmentBase64(args ?? {}),
     copy_attachment_file: (args) => copyAttachmentFileToManagedStorage(args ?? {}),