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>

package.json

@@ -22,6 +22,10 @@
     "release": "pwsh -NoProfile -ExecutionPolicy Bypass -File scripts/publish.ps1 -PublishRelease",
     "test": "node --test \"test/**/*.test.js\""
   },
+  "dependencies": {
+    "@ffmpeg/core-st": "^0.11.1",
+    "@ffmpeg/ffmpeg": "^0.11.6"
+  },
   "devDependencies": {
     "electron": "^40.0.0",
     "electron-builder": "^26.0.0"
@@ -38,12 +42,23 @@
       "electron/**/*",
       "scripts/server.cjs",
       "src/**/*",
-      "!src/**/*.map"
+      "!src/**/*.map",
+      "!node_modules/@ffmpeg/**"
     ],
     "win": {
       "target": [
-        { "target": "nsis", "arch": ["x64"] },
-        { "target": "portable", "arch": ["x64"] }
+        {
+          "target": "nsis",
+          "arch": [
+            "x64"
+          ]
+        },
+        {
+          "target": "portable",
+          "arch": [
+            "x64"
+          ]
+        }
       ],
       "icon": "build/icon.png"
     },
@@ -55,7 +70,10 @@
       "shortcutName": "Motionity"
     },
     "linux": {
-      "target": ["AppImage", "flatpak"],
+      "target": [
+        "AppImage",
+        "flatpak"
+      ],
       "icon": "build/icon.png",
       "category": "Graphics",
       "synopsis": "Motion graphics editor",