refactor: unify per-platform dispatch in window helpers via cfg_select!

[StudentWeis]

Current State

src/gui/utils.rs::hide_window and src/gui/utils.rs::active_window both organise their per-platform code as two adjacent blocks:

// 1. Windows path written as a standalone #[cfg] block
#[cfg(target_os = "windows")]
if let Ok(window_handle) = HasWindowHandle::window_handle(window)
    && let RawWindowHandle::Win32(win32_handle) = window_handle.as_raw()
{
    let hwnd = win32_handle.hwnd.get() as *mut std::ffi::c_void;
    unsafe { /* ShowWindow / SetForegroundWindow */ }
}

// 2. macOS / Linux paths written through cfg_select!
cfg_select! {
    target_os = "macos" => { /* … */ }
    target_os = "linux" => { /* … */ }
    _ => {}
}

The split is purely historical: the Windows block was written with a multi-line unsafe body and a local hwnd binding, while the other branches were one-liners that fit naturally into cfg_select!. There is no language reason for the asymmetry — cfg_select! arms accept arbitrary statement blocks, including unsafe { … } and local let bindings.

Target State

Each helper has a single per-platform dispatch site:

cfg_select! {
    target_os = "macos" => { /* … */ }
    target_os = "windows" => {
        if let Ok(window_handle) = HasWindowHandle::window_handle(window)
            && let RawWindowHandle::Win32(win32_handle) = window_handle.as_raw()
        {
            let hwnd = win32_handle.hwnd.get() as *mut std::ffi::c_void;
            unsafe { /* … */ }
        }
    }
    target_os = "linux" => { /* … */ }
    _ => {}
}

No platform-specific logic is added or removed. Both the X11 map_window ↔ unmap_window pairing and the Win32 SW_RESTORE ↔ SW_HIDE pairing (see issue #101) are preserved verbatim — they are the project's bridges between its own hide path and GPUI's activation path, and must stay.

Rationale

• Consistency — the same dispatch shape across all three platforms makes the function easier to read at a glance.
• DRY — one cfg dispatch site per function instead of two.
• No behaviour change — cfg_select! expands to the same compile-time branch selection as #[cfg], so the binary is byte-identical on each target.
• Risk is essentially zero; clippy backstops any unused-import fallout.

Scope

gui (style cleanup inside two helpers in src/gui/utils.rs).

Safety Net

• Existing tests cover the affected behavior — pure refactor, runtime semantics unchanged.
• New tests will be added as part of this refactor — N/A.
• This refactor is purely mechanical (no behavior change).