Firebase Admin SDK file configuration is not portable across environments due to stored file entity IDs

Problem/Motivation

The module currently stores the uploaded Firebase Admin SDK JSON file as a file entity ID (`fid`) in configuration (`firebase_ui.settings`).

Since file entity IDs are environment-specific, this configuration is not portable across environments. After configuration import, the stored `fid` may point to:
- a non-existing file,
- a different file,
- or an invalid file entity.

This causes Firebase authentication failures and runtime errors when deploying configuration between environments.

Steps to reproduce

1. Configure Firebase UI in a local environment.
2. Upload a Firebase Admin SDK JSON file in:
`/admin/config/services/firebase-ui`
3. Export configuration.
4. Import configuration into another environment.
5. The imported `fid` references an invalid or unrelated file entity.
6. Firebase features fail due to invalid credentials.

Proposed resolution

Follow what was done in #3462829: Store the file path instead of ID for the navigation logo

Remaining tasks

User interface changes

API changes

None

Data model changes