fix: stop telling Linux and Windows they are a Mac

[itz4blitz]

Summary

The Linux build of 0.2.5 shows Mac copy: "Your sign-in is kept in the macOS keychain on this Mac.", "Show in Finder", and "Choose Always Allow". Those sentences were compiled in for every system.

host now names the computer this build runs on.

• macOS stays on the keychain, Finder, and Always Allow.
• Linux says the system keyring, "this computer", and "Show in folder". Copied diagnostics use PRETTY_NAME from os-release instead of a sw_vers line that never appears.
• Windows says Credential Manager, "this PC", and File Explorer.
• Opening a call link on Linux uses xdg-open, the same as opening a file already does.

Test plan

• cargo test -p tinyclient host:: on Linux, macOS, and Windows (CI runs this on all three)
• Linux welcome screen says the system keyring, not the macOS keychain
• Settings storage and logs say "Show in folder"
• macOS welcome screen still says the macOS keychain