Add desktop/server MP4 export pipeline, hardening, and render tests (v1.3.0)

Desktop MP4 export renders frames from a hidden BrowserWindow via
capturePage, pipes PNGs to a bundled ffmpeg (libx264/yuv420p/+faststart),
and writes the file through an atomic temp-file swap. It shares scene
validation (shared/renderValidation.cjs) and the render.html renderer with
the standalone Puppeteer server, which is hardened in parallel.

Render worker & main (electron/):
- abortable render loop with an inactivity watchdog and cancel plumbing
- atomic temp-file + rename so a cancel/failure never truncates a
  pre-existing target (#7); stdin EPIPE / early-exit / backpressure-hang
  handled (#8)
- HiDPI frames normalized to canvas size (#10); odd dimensions padded to
  even (#13, also server); backgroundThrottling disabled (#12)
- reject canvases larger than the display (#11); single-render guard;
  isolated render session that blocks network requests (#5)

Tests:
- Electron harnesses (scripts/verify-*.cjs) for capture, export, cancel,
  timeout, assets, background, capture-fidelity, resolution, odd-dims,
  aggregated by verify-desktop-render.cjs
- server smoke + e2e and shared validation unit tests
- headless hook (src/devtools/headless.ts) to run the in-app suite under
  browser/electron hosts; pixel-diff util + capture fixtures (test/)
- dev_docs plans: integration, fixes (#7-#13), and the e2e/capture strategy

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -68,6 +68,18 @@ npm run desktop:build  # build platform installer/package artifacts into release
 app without creating an installer. `desktop:build` uses `electron-builder` and
 targets AppImage on Linux, NSIS on Windows, and DMG on macOS.
 
+Desktop MP4 export runs inside Electron and uses ffmpeg from
+`resources/bin/<platform>/` in packaged builds. The build scripts run
+`npm run stage:ffmpeg -- --optional`, which copies `FFMPEG_PATH` or `ffmpeg` on
+`PATH` into `bin/<platform>/` when available. For release artifacts, replace
+that local copy with redistributable static ffmpeg binaries for Linux, Windows,
+and macOS.
+
+If no bundled binary is present, MP4 export still falls back to `FFMPEG_PATH`
+or system `ffmpeg` and reports a clear error when neither can be executed.
+Before shipping bundled ffmpeg, document the binary source, version, configured
+codecs, and GPL/LGPL obligations for that build.
+
 ## MP4 render server
 
 MP4 export is handled by the optional render server in `server/`. It binds to
@@ -96,6 +108,16 @@ The included `docker-compose.yml` keeps the published host port on
 `127.0.0.1:6000` and requires `RENDER_TOKEN` in the environment before it will
 start.
 
+Render-server checks:
+
+```bash
+npm run test:render-server      # health/auth/validation route smoke
+npm run test:render-server:e2e  # renders a tiny MP4 and verifies it with ffprobe
+```
+
+The E2E check requires Chromium, ffmpeg, and ffprobe. Set
+`PUPPETEER_EXECUTABLE_PATH` when Chromium is not at `/usr/bin/chromium`.
+
 ## Stack
 
 - Vue 3 (`<script setup>`) + Pinia state