Move from keytar to Electron's safeStorage API.

[bajtos]

keytar is no longer maintained as of Dec 2022, see atom/node-keytar#482.

Other projects like VS Code solved the problem by moving to Electron's safeStorage API, see https://code.visualstudio.com/updates/v1_80#_secretstorage-api-now-uses-electron-api-over-keytar

We should follow their lead and to the same.

Example pull request for inspiration: jfrog/jfrog-vscode-extension#417

We must be extra careful when implementing this change, since we use keytar to store the wallet seed. If we mess this up, the user won't have access to their wallet & rewards earned.