chore: release v0.12.0

Bump to 0.12.0 and record the changelog: in-dialog send-to-wallet stepper,
past-date-blocking appointment/invoice pickers, wallet-number-only portal link,
and removal of the stale Settings Features section.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "frontend",
-  "version": "0.11.0",
+  "version": "0.12.0",
   "private": true,
   "scripts": {
     "dev": "next dev",