Add address fields, e-invoice, stats and auto-backup

- Split address into street, postal code, city (sortable)
- Add billing postal code/city fields
- Add e-invoice address and operator fields
- Add trivia stats (top postal code, top speed, avg price)
- Improved layout with stat cards grid and max-width container
- Sticky header, modal animations, search icon
- Auto-backup on every save (keeps last 30 backups)
- Footer added

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1 +1,2 @@
 data/customers.json
+data/backups/