Add welcome screen when the app starts up without a vault set

[fmaclen]

• Replace the native OS components with a browser-based UI that is themed like the rest of the app.
• This process might involve implementing a 2-way communication channel between Electron and SvelteKit to trigger the right file chooser events.
• An easier implementation could be to duplicate some of the UI assets (css, icons, fonts, etc) from SvelteKit and use them directly in a BrowserWindow in Electron.