Save and switch between multiple Vintage Story accounts

[Zaldaryon]

Is your feature request related to a problem? Please describe.
RiftLauncher currently supports one active Vintage Story account. On a shared device, a second player has to log out and enter their credentials again whenever they want to play with their own account. This is inconvenient for households and other shared-device setups.

Describe the solution you'd like
Allow users to save multiple Vintage Story accounts in RiftLauncher and select which one is active before launching the game. The launcher should keep each account's session data in secure storage and switch the active session without requiring a full login every time. It should not store passwords in plaintext.

Describe alternatives you've considered
The current workaround is to log out and log back in for every user. Separate operating-system accounts or separate launcher data directories would also isolate sessions, but they are less convenient for people sharing one device.

Additional context
The current account flow supports only one account:

• src/ipc/accountStore.ts stores all secrets in a single account-secrets.json; saving replaces that file and logout removes it.
• ConfigType exposes a single account: AccountPublicType | null.
• The preload API exposes only login and logout, and SessionButton presents either the current account or the login/logout flow.

Could this be considered as a feature request?

[Pixnop]

Shipped in #253, with the store-preservation companion #261. Cross-account session adoption is pinned at the handler level and the single-account migration retries on a locked keyring.