feat: receipt customization settings tab with header, footer, policy, social

- New Receipt tab in Settings page with editable fields
- receipt_header: text below logo (e.g. tagline)
- receipt_footer: thank you message
- receipt_return_policy: return policy text
- receipt_social: website/social media
- All stored in app_config, rendered on printed receipts
- Seeded in migration with empty defaults

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

packages/backend/src/db/migrations/0042_user-pin.sql

@@ -10,7 +10,11 @@ BEGIN
   END LOOP;
 END $$;
 
--- Seed POS lock timeout config
-INSERT INTO "app_config" ("key", "value", "description")
-VALUES ('pos_lock_timeout', '15', 'POS auto-lock timeout in minutes (0 to disable)')
+-- Seed POS config
+INSERT INTO "app_config" ("key", "value", "description") VALUES
+  ('pos_lock_timeout', '15', 'POS auto-lock timeout in minutes (0 to disable)'),
+  ('receipt_header', '', 'Text shown below logo on receipts'),
+  ('receipt_footer', '', 'Thank you message at bottom of receipt'),
+  ('receipt_return_policy', '', 'Return policy text on receipt (blank to hide)'),
+  ('receipt_social', '', 'Website or social media shown on receipt')
 ON CONFLICT ("key") DO NOTHING;