fix: clean up orphaned Chrome and ffmpeg processes on preview exit

The preview command's shutdown handler only closed the HTTP server,
leaving Chrome (browser pool) and ffmpeg processes alive. This caused
silent resource leaks — orphaned processes consuming CPU and RAM with
no parent.

Root cause: preview.ts never called drainBrowserPool() or killed
tracked ffmpeg processes. The thumbnail browser in studioServer.ts
registered its own competing signal handlers that raced with
preview's shutdown.

Fix:
- Add a central process tracker (processTracker.ts) that registers
  every spawned ffmpeg across engine and producer packages
- Centralize thumbnail browser cleanup via exported
  closeThumbnailBrowser() instead of scattered signal handlers
- Wire preview shutdown to call closeThumbnailBrowser(),
  drainBrowserPool(), and killTrackedProcesses() before closing the
  HTTP server (embedded mode)
- Add killProcessTree() for dev/local modes where Chrome runs in a
  child process tree
- Add startup orphan detection that finds and kills orphaned
  chrome-headless-shell/Puppeteer Chrome processes (PPID=1) from
  previously crashed sessions

Closes #1038

packages/engine/src/utils/runFfmpeg.ts

@@ -6,6 +6,7 @@
  */
 
 import { spawn } from "child_process";
+import { trackChildProcess } from "./processTracker.js";
 
 export interface RunFfmpegOptions {
   signal?: AbortSignal;
@@ -60,6 +61,7 @@ export async function runFfmpeg(args: string[], opts?: RunFfmpegOptions): Promis
 
   return new Promise<RunFfmpegResult>((resolve) => {
     const ffmpeg = spawn("ffmpeg", args);
+    trackChildProcess(ffmpeg);
     let stderr = "";
 
     const onAbort = () => {