fix(desktop): gate macOS title_bar_style behind cfg for Linux builds

[jrtorrez31337]

Summary

Saw issue #802 (unable to add API keys on Linux) and started looking into the Linux desktop build situation. The desktop app doesn't compile on Linux because WebviewWindowBuilder::title_bar_style() is a macOS-only Tauri API. Two call sites in main.rs use it unconditionally.

Can only verify Linux for now — don't have a macOS or Windows build environment to confirm those are unaffected.

Changes

• Split the builder chains for settings and live-channels windows so title_bar_style(TitleBarStyle::Overlay) is only called behind #[cfg(target_os = "macos")]
• Added #[allow(unused_mut)] to suppress the resulting warning on non-macOS targets where the mut binding is only needed for the cfg-gated reassignment

Test plan

• cargo tauri build succeeds on Linux (verified in Ubuntu 24.04 container)
• macOS build still gets the overlay title bar style (not verified, no macOS environment)
• Windows build unaffected (not verified)

[vercel]

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