refactor(main): execFileAsync helper — unify the 5 spawn-and-read sites

Replace the hand-rolled `new Promise((resolve) => execFile(…, cb))` wrappers in
probe / ytdlp (×2) / ffmpeg / indexer.probeFlat / download.probeMeta with one
lib/exec.ts execFileAsync returning a normalized
{ok,stdout,stderr,timedOut,error} that never rejects. Each caller keeps its own
error mapping (timedOut -> "Timed out …" vs stderr) and JSON parsing; behaviour
is unchanged. windowsHide defaults on; per-call timeout/maxBuffer preserved.

Foundation for CC4/CC5 — the stdout line-buffer half lands with SIMP5, the
net.request half with SIMP16, at which point those checkboxes close.

Unit-tested (exec.test.ts) against the node binary: ok / stderr+nonzero-exit /
missing-binary / timeout. typecheck + 262 tests + eslint + prettier green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/ffmpeg.ts

@@ -1,6 +1,6 @@
-import { execFile } from 'child_process'
 import { existsSync } from 'fs'
 import { getFfmpegPath, getFfprobePath } from './binaries'
+import { execFileAsync } from './lib/exec'
 import { VERSION_TIMEOUT_MS } from './constants'
 import type { FfmpegVersionResult } from '@shared/ipc'
 
@@ -12,24 +12,13 @@ import type { FfmpegVersionResult } from '@shared/ipc'
  * out, or the line doesn't parse — Settings then shows "not found" instead of
  * failing. Unlike yt-dlp these are never self-updated, so there's no update path.
  */
-function readToolVersion(path: string): Promise<string | null> {
-  if (!existsSync(path)) return Promise.resolve(null)
-  return new Promise((resolve) => {
-    execFile(
-      path,
-      ['-version'],
-      { windowsHide: true, timeout: VERSION_TIMEOUT_MS },
-      (err, stdout) => {
-        if (err) {
-          resolve(null)
-          return
-        }
-        const firstLine = stdout.split('\n', 1)[0] ?? ''
-        const m = firstLine.match(/version\s+(\S+)/i)
-        resolve(m?.[1] ?? null)
-      }
-    )
-  })
+async function readToolVersion(path: string): Promise<string | null> {
+  if (!existsSync(path)) return null
+  const r = await execFileAsync(path, ['-version'], { timeout: VERSION_TIMEOUT_MS })
+  if (!r.ok) return null
+  const firstLine = r.stdout.split('\n', 1)[0] ?? ''
+  const m = firstLine.match(/version\s+(\S+)/i)
+  return m?.[1] ?? null
 }
 
 /** Versions of the bundled ffmpeg + ffprobe, read in parallel for the Settings panel. */