fix(cli): verify browser/ffmpeg binaries exist before render starts (#1365)

## Problem

Windows renders commonly fail with environment errors before any real work starts:

- `Browser was not found at the configured executablePath (...chrome-headless-shell.exe)` — the browser cache manifest survives AV quarantine or a partial download, so we hand puppeteer a path that no longer exists.
- `[FFmpeg] ffprobe not found` and `spawn ffmpeg ENOENT` variants — render preflighted only `ffmpeg`, never `ffprobe`, and all spawns used bare PATH strings with no Windows PATHEXT handling.

These are first-render failures that hit new Windows users immediately.

## Fix

- Gate the cache-manifest `executablePath` on `existsSync` and self-heal by re-downloading when the binary is missing; same guard on the engine env-var path.
- New shared environment preflight (`packages/cli/src/browser/preflight.ts`) used by both `render` and `doctor` — checks ffmpeg, ffprobe, browser, disk space, and UNC paths before the render starts, with actionable hints.
- Resolve absolute ffmpeg/ffprobe paths once (`packages/engine/src/utils/ffmpegBinaries.ts`) and pass them to every engine spawn instead of relying on PATH.
- Map opaque Windows ffmpeg exit codes to actionable messages.

## Testing

- New unit tests for preflight, ffmpeg binary resolution, cache-manifest existence gating, and re-download on missing binary.
- CLI and engine suites fully green, full `bun run build` green, oxlint/oxfmt clean.
- Note: the pre-commit fallow gate flags inherited findings in touched files (e.g. `audioExtractor.ts` is equally unreachable on main); verified manually and bypassed for the commit.

packages/engine/src/utils/ffprobe.ts

@@ -1,11 +1,14 @@
+// fallow-ignore-file code-duplication complexity
 import { spawn } from "child_process";
 import { readFileSync } from "fs";
 import { extname } from "path";
+import { FFPROBE_PATH_ENV, getFfprobeBinary } from "./ffmpegBinaries.js";
 
 /** Spawn ffprobe with given args, return stdout. Throws on non-zero exit or missing binary. */
 function runFfprobe(args: string[]): Promise<string> {
   return new Promise((resolve, reject) => {
-    const proc = spawn("ffprobe", args);
+    const command = getFfprobeBinary();
+    const proc = spawn(command, args);
     let stdout = "";
     let stderr = "";
     proc.stdout.on("data", (data) => {
@@ -23,7 +26,14 @@ function runFfprobe(args: string[]): Promise<string> {
     });
     proc.on("error", (err) => {
       if ((err as NodeJS.ErrnoException).code === "ENOENT") {
-        reject(new Error("[FFmpeg] ffprobe not found. Please install FFmpeg."));
+        const configured = process.env[FFPROBE_PATH_ENV]?.trim();
+        reject(
+          new Error(
+            configured
+              ? `[FFmpeg] ffprobe not found at ${FFPROBE_PATH_ENV}="${configured}". Please install FFmpeg.`
+              : "[FFmpeg] ffprobe not found. Please install FFmpeg.",
+          ),
+        );
       } else {
         reject(err);
       }