Fix config.toml overwritten on update

[h30s]

Fixes #1

install.sh and install.ps1 unconditionally regenerated config.toml with default template values on every install and update, wiping configured [features], resetting the chosen model, and reverting model_provider = "byok-local" or subscription back to byteask.

This guards config.toml generation so default settings are only written when config.toml does not already exist, preserving existing user configuration on updates while safely inserting model_catalog_json if upgrading from an older release lacking it.

Tested:

• added regression test tests/test_install.sh (fails before, passes after)
• ran tests/test_install.sh, sh -n install.sh, sh -n byteask
• ran the reproduction script from the issue