feat: replace the archive.org asm.js encoder with vendored ffmpeg.wasm

MP4/GIF export used to importScripts() an 18.5 MB asm.js ffmpeg build from
https://archive.org/download/ffmpeg_asm/ffmpeg_asm.js: no integrity check, no
pinning, executed in the page, and unavailable offline. vendor.mjs now copies
ffmpeg.wasm out of node_modules, where package-lock.json pins it by hash, and
no CDN fallback is left anywhere in the app.

@ffmpeg/core-st is the single-threaded core, chosen deliberately: the default
@ffmpeg/core is built with pthreads and needs SharedArrayBuffer, which requires
COOP/COEP isolation, which would break the Pixabay, Unsplash and Google Fonts
requests. That core also forces two things worth knowing:

- mainName: 'main' is mandatory. The loader defaults to proxy_main, which only
  the multi-threaded build exports, so load() compiles all 23 MB and then aborts.
- Its main() calls exit(), so an instance survives exactly one command. Reusing
  one dies with "Program terminated with exit(0)", so convertStreams builds and
  tears one down per conversion (~110 ms, and the 23 MB heap comes back in
  between). The teardown also runs on failure: an interrupted run otherwise
  leaves the loader's "running" flag set and wedges every later conversion until
  a page reload.

MP4 encodes with libx264 -crf 23 -pix_fmt yuv420p plus AAC rather than
mpeg4 -b:v 6400k. Same core, better quality per byte, and yuv420p is what makes
it play in Safari and QuickTime.

The two @ffmpeg packages are dependencies, not devDependencies, so the Docker
vendor stage can npm ci --omit=dev without pulling in electron; build.files
excludes them from the asar since src/vendor/ffmpeg/ already carries the copies
the app loads. WITH_FFMPEG=0 now means MP4/GIF export is unavailable and says
so, rather than silently fetching an encoder at run time.

Also deletes src/js/libraries/ffmpeg.min.js, an unreferenced ffmpeg.wasm loader
stub that would have fetched its core from unpkg, and prunes the stale
src/vendor/ffmpeg_asm.js from existing checkouts — src/vendor/ is packaged
whole, so it would have shipped 18.5 MB of dead weight in every installer.

Verified in Chromium against a real MediaRecorder WebM: core loads with
crossOriginIsolated false, MP4 24 KB decoding to 320x240 / 2.00 s, GIF 138 KB,
the two back to back, and the missing-core path reporting correctly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -6,13 +6,19 @@
 # Build:  docker build -t motionity:latest .
 # Run:    docker run --rm -p 8080:8080 motionity:latest
 #
-# WITH_FFMPEG=0 drops the 18.5 MB asm.js ffmpeg build from the image. MP4/GIF
-# export then downloads it from archive.org on first use instead of working
-# offline; everything else is unaffected.
+# WITH_FFMPEG=0 drops the 23 MB ffmpeg.wasm core from the image and disables
+# MP4/GIF export, which then reports itself as unavailable. It no longer falls
+# back to downloading the encoder at run time: the old asm.js build came from a
+# public archive.org mirror with no integrity check. Everything else, WEBM
+# export included, is unaffected.
 
 FROM node:22-alpine AS vendor
 ARG WITH_FFMPEG=1
 WORKDIR /app
+# ffmpeg.wasm is a runtime dependency, so --omit=dev pulls it in without
+# dragging electron and electron-builder (~400 MB) into the build.
+COPY package.json package-lock.json ./
+RUN npm ci --omit=dev --no-audit --no-fund
 COPY scripts/vendor.mjs scripts/
 COPY src/index.html src/
 RUN if [ "$WITH_FFMPEG" = "1" ]; then \