Establish the Linux native build foundation

[enaboapps]

Implement the first Linux foundation increment from #710 / #711 as the first implementation PR in a stack.

• Make the Rust/Tauri Linux target compile with an explicitly unavailable transport/input runtime.
• Keep unsupported overlays from preventing startup; retain a reachable main window without depending on a tray host.
• Add Ubuntu native lint, tests and development build validation, using fake input only.
• Preserve Windows/macOS behavior and protocol/persistence compatibility.

This is a development foundation, not working Bluetooth or input support. Follow with a stacked PR for Linux setup/status UX and feature gating, then the BlueZ interoperability milestone.

[enaboapps]

Closing as completed: the scoped work was merged in #714 into linux-support. This records completion on that development branch; it does not imply the work is in main or released.