License the project under AGPL-3.0-or-later

Adds the verbatim license text as LICENSE, sets license metadata in
both package manifests, and links it from both README.md files.
Chosen specifically for the network-copyleft clause (AGPL §13): a
modified version run as a hosted service must offer its source to
that service's users, which plain GPL's distribution-only trigger
doesn't cover.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/package.json

@@ -2,6 +2,7 @@
   "name": "frontend",
   "private": true,
   "version": "0.0.0",
+  "license": "AGPL-3.0-or-later",
   "type": "module",
   "scripts": {
     "dev": "vite",