Linux desktop install path for TytusOS

[rschumann]

Follow-up from TytusOS tray cutover PR #11.

Current install matrix is intentionally honest:

• macOS: tytus tray install installs /Applications/Tytus.app and opens embedded TytusOS.
• Linux: CLI/auth/daemon/MCP supported, but desktop tray/browser packaging is not yet shipped.

Task:

1. Decide Linux desktop mode: AppIndicator tray app, .desktop launcher that opens the local TytusOS URL, or both.
2. Add installer support for the chosen mode.
3. Add systemd user service + log/status docs for daemon/tray web startup.
4. Add manual QA on Ubuntu + Fedora/GNOME.

Acceptance:

• Fresh Linux user can run the install command, authenticate, and open TytusOS without guessing commands.
• If tray is unavailable on a distro/session, installer prints a browser fallback command.