fix(app): only show startup info modal on initial root load

[antontranelis]

Fixes follow-up after #857.

The startup info modal is now evaluated only once when ModalContent mounts. It opens only if the initial route was /, so navigating back to / from /set-new-password or other in-app routes no longer pops the welcome dialog.

Verification:

• npm --workspace app run build
• ../node_modules/.bin/eslint src/ModalContent.tsx from app/
• git diff --check

Not run:

• Cypress E2E, because local app/backend were not running on http://localhost:8080 / http://localhost:8055.
• npm --prefix cypress run lint, because local cypress dependencies are incomplete: eslint-plugin-cypress is missing.