Warn users about Flatpak/Snap on distros that support native packages

Since we recommend our native packages over the Flatpak and Snap, we should warn users if we detect that they are installed on distros where native packages are available.

We should also provide instructions on how to remove them (without trashing users' browser profiles).

No file or test is named in the issue. Start by reading the installer entry point and its existing package-detection logic, then check how Flatpak and Snap installations can be identified across supported distributions. Done means warning only where native packages are available and providing removal instructions that preserve browser profiles.

Written by the indexing model from the issue text.