docs: renewed README and light/dark diagonal screenshots; wider hashtag tiles

- README: current features (journal, focus mode, push, themes, sharing
  into the inbox), outgoing connections, :latest instead of :stable,
  VAPID_* and LEGAL_CUSTOM_LINK_*, new journal and phone screenshots
- e2e/screenshots.sh: light and dark in the same state joined along a
  diagonal, own container/port per run
- hashtag tiles at least 18rem wide

chore(release): v0.15.0

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "mindsift-frontend",
   "private": true,
-  "version": "0.14.3",
+  "version": "0.15.0",
   "license": "Unlicense",
   "type": "module",
   "scripts": {