Add authentication and date picker to Receipt Manager

Add username/password login with session cookies, change-password modal,
and logout. All API endpoints require a valid session. Default credentials
(admin/admin) are auto-created on first run. Also add a date input field
to the receipt modal, defaulting to today but allowing the user to pick
any date.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,7 @@
+services:
+  receipt-manager:
+    build: .
+    container_name: receipt-manager
+    restart: always
+    ports:
+      - "8082:8080"