settings: add UI options to import/export subscriptions

[AndrianaBilali]

What is it?

• [ ] Bugfix (user facing)
• [x] Feature (user facing)
• [ ] Codebase improvement (dev facing)
• [ ] Meta improvement to the project (dev facing)

Description of the changes in your PR

Adds two new options to the Backup & Restore settings screen:

• "Export subscriptions": saves current subscriptions as a .json file
• "Import subscriptions": restores subscriptions from a previously exported .json file

Reuses existing import/export logic via SubscriptionsImportService and SubscriptionsExportService. These are 2 .json files created for testing the feature:

Before/After Screenshots/Screen Record

Before	After

Fixes the following issue(s)

• Fixes #12026

Due diligence

• [x] I read the contribution guidelines.

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud