docs(readme): new hero screenshot (1400×862, 289 KB)

Creator-supplied capture of the main window: five scenes across the top, the Starting scene with its
three callouts in the preview, the layer panel, and the audio dock.

Re-encoded with the bundled ffmpeg instead of the WPF resize tool (scale 1400 + q4) — one command, the
same tool the rest of the assets use — and that shortcut is recorded in MyMistakes next to the original
recipe so it is not re-derived a third time.

MyMistakes.md

@@ -589,6 +589,17 @@ packages, high-quality downscale via `TransformedBitmap`. Screenshots compress
 
 **Result:** 3.2MB screenshot → 1400×794 → **188KB** `ytLlive-preview.jpg` in `docs/`.
 
+**Faster route (2026-11-09): the bundled ffmpeg does the same job in one command** — no throwaway
+project, and it is already the tool used for every other asset:
+
+```bash
+./ffmpeg/ffmpeg.exe -hide_banner -loglevel error -y -i "C:\path\shot.png" \
+  -vf "scale=1400:-2:flags=lanczos" -q:v 4 "C:\...\docs\ytLlive-preview.jpg"
+```
+
+`-q:v 4` is the sweet spot for UI screenshots (text stays crisp); q3 runs ~340 KB, q6 ~230 KB.
+The WPF recipe above stays as the fallback if ffmpeg is unavailable.
+
 ---
 
 ### Measuring audio-video A/V sync from a recording (no ears needed) — RECIPE