Bundle ffprobe.exe and complete the real-download smoke-test pass

A live smoke test of buildArgs' argv against the bundled yt-dlp + ffmpeg found
that ffprobe.exe was never bundled (resources/bin/ shipped only ffmpeg.exe).
yt-dlp resolves both from --ffmpeg-location, so duration-aware post-processing
failed at runtime for every user with "ffprobe not found": --sponsorblock-remove,
--force-keyframes-at-cuts, and --split-chapters (CODE-AUDIT C1).

- Bundle ffprobe.exe (matching n8.1.2 LGPL build, sha256-verified against the
  already-bundled ffmpeg.exe) and document it in resources/bin/README.md as a
  required binary; correct the stale "not committed to git" note.
- Guard startDownload against a missing ffmpeg.exe/ffprobe.exe up front so the
  failure is a clear AeroFetch error, not a cryptic yt-dlp postprocessing one
  (new getFfprobePath() in binaries.ts).
- Expand test/real-download.integration.test.ts from 2 to 8 live cases: crop,
  sponsorblock-remove, audio opus re-encode, mkv+vp9 merge, subtitle+chapter
  embed, restrict-filenames, download-archive skip, and Phase C extra-args.
  All 8 pass against live yt-dlp + ffmpeg.
- ROADMAP: mark the Phase A/B/C smoke-test caveats done. CODE-AUDIT: add C1.

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

src/main/binaries.ts

@@ -21,6 +21,16 @@ export function getFfmpegPath(): string {
   return join(getBinDir(), 'ffmpeg.exe')
 }
 
+/**
+ * yt-dlp finds ffprobe via --ffmpeg-location (the bin dir), so the app never
+ * spawns it directly — but it must be present, or duration-aware post-processing
+ * (SponsorBlock-remove, --force-keyframes-at-cuts, --split-chapters) fails. This
+ * accessor exists so startDownload can assert its presence up front.
+ */
+export function getFfprobePath(): string {
+  return join(getBinDir(), 'ffprobe.exe')
+}
+
 /** Optional bundled external downloader; absent unless dropped into resources/bin. */
 export function getAria2cPath(): string {
   return join(getBinDir(), 'aria2c.exe')

src/main/download.ts

@@ -2,7 +2,7 @@ import { spawn, execFile, type ChildProcess } from 'child_process'
 import { existsSync } from 'fs'
 import { join } from 'path'
 import { app, BrowserWindow, Notification, type WebContents } from 'electron'
-import { getYtdlpPath, getBinDir, getAria2cPath } from './binaries'
+import { getYtdlpPath, getBinDir, getAria2cPath, getFfmpegPath, getFfprobePath } from './binaries'
 import { getSettings, getDownloadArchivePath } from './settings'
 import { getCookiesFilePath } from './cookies'
 import { listTemplates } from './templates'
@@ -213,6 +213,22 @@ export function startDownload(
       error: `yt-dlp.exe not found at ${ytdlp}\nDrop it into resources/bin/ (see the README there).`
     }
   }
+  // ffmpeg is used by nearly every download (merge, audio extract, thumbnail/
+  // metadata embed) and ffprobe by duration-aware post-processing (SponsorBlock-
+  // remove, --force-keyframes-at-cuts, --split-chapters). Assert both up front so a
+  // missing binary is a clear AeroFetch message, not a cryptic mid-download yt-dlp
+  // postprocessing error (e.g. "Unable to determine video duration: ffprobe not found").
+  const missingBins: string[] = []
+  if (!existsSync(getFfmpegPath())) missingBins.push('ffmpeg.exe')
+  if (!existsSync(getFfprobePath())) missingBins.push('ffprobe.exe')
+  if (missingBins.length > 0) {
+    return {
+      ok: false,
+      error:
+        `${missingBins.join(' and ')} not found in ${getBinDir()}\n` +
+        `Add the ffmpeg build's binaries to resources/bin/ (see the README there).`
+    }
+  }
   // Reject anything that isn't an http(s) URL before it reaches yt-dlp's argv.
   try {
     assertHttpUrl(opts.url)