feat: integrate Cal.com popup booking via @calcom/embed-react

- CalButton component: loads Cal.com embed script, triggers popup on click
- Configured for scheduling.letsbe.solutions (self-hosted)
- Wired into configurator completion step (Book a Call)
- Wired into footer Book a Call button
- Brand colors: #006494 light, #5BA4D9 dark

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -10,6 +10,7 @@
     "generate:types": "payload generate:types"
   },
   "dependencies": {
+    "@calcom/embed-react": "^1.5.3",
     "@payloadcms/db-postgres": "^3.80.0",
     "@payloadcms/next": "^3.80.0",
     "@payloadcms/richtext-lexical": "^3.80.0",