feat(ui): connection status in the header; README screenshots as WebP

- Desktop shows "offline" with a red dot under the user name when the
  server is unreachable; phones show a green/red dot on the avatar.
  Based on the app-wide realtime connection, so a server outage is
  noticed even while the device has network (4 s grace period).
- Screenshots converted to WebP (1280 px desktop, 480 px phone, q90),
  e2e/screenshots.sh converts with cwebp; new journal picture with
  round corners. Rules in AGENTS.md.
- Release v0.26.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "mindsift-frontend",
   "private": true,
-  "version": "0.25.1",
+  "version": "0.26.0",
   "license": "Unlicense",
   "type": "module",
   "scripts": {