mobile: definitive slide fix (zero teardown) + real native download + reconcile (batch125)

Slide: the residual ~40ms end-of-slide hitch (down from 128ms in b124) was the layer
DEMOTE + the deferred full renderThread at transitionend. Two fixes:
- Keep .content promoted for the WHOLE time a chat is open (will-change lives on
  body.chat-open, not on the animating class). The open transition now only changes the
  transform VALUE 100%->0; when it ends there is nothing to tear down -> no demote raster.
- Slide-end no longer rebuilds all 40 nodes: reconcileOpen() appends only the 0-2 genuinely
  new tail messages (full renderThread only if the page structure diverged).

Download: routing to Safari failed auth (no login cookie). Real fix: fetch the file in the
WebView (cookie present) and hand the bytes to the OS save/share sheet via Filesystem+Share
(added to mobile deps; ships next TestFlight build). Until then, images fall back to the iOS
long-press "Save to Photos" instead of breaking the app.

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

mobile/package.json

@@ -15,8 +15,10 @@
     "@capacitor/app": "^7.0.0",
     "@capacitor/camera": "^7.0.0",
     "@capacitor/core": "^7.0.0",
+    "@capacitor/filesystem": "^7.0.0",
     "@capacitor/ios": "^7.0.0",
     "@capacitor/keyboard": "^7.0.0",
+    "@capacitor/share": "^7.0.0",
     "@capacitor/push-notifications": "^7.0.0",
     "@capacitor/splash-screen": "^7.0.0",
     "@capacitor/status-bar": "^7.0.0"