fix(preview): set GTK WindowTypeHint::Utility for non-KDE compositor coverage

KWin reads _NET_WM_STATE_SKIP_TASKBAR for its Alt+Tab list, which OW-2
already wired via skip_taskbar(true) on the builder. On Hyprland, Sway,
and GNOME Mutter that's not always enough — some switchers still enumerate
the overlay. Classifying the window as gdk::WindowTypeHint::Utility signals
to the compositor that this is an assistive auxiliary surface, so switchers
and auto-tilers leave it alone. No behavioural change on KWin.

GTK3 only honours the type hint before the window maps, so the preview
builder now starts .visible(false); we grab the gtk_window() via Tauri's
escape hatch, set the hint, then show(). The existing hide/show on
re-open still works — hint is a property of the gtk::ApplicationWindow
and survives the cycle.

Added gtk = "0.18" and gdk = "0.18" as Linux-only deps. Both are already
pulled in transitively via webkit2gtk 2.0, so this is just surfacing them
by name — no new compile cost.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -46,3 +46,8 @@ uuid = { version = "1", features = ["v4"] }
 
 [target.'cfg(target_os = "linux")'.dependencies]
 webkit2gtk = "2.0"
+# Needed for setting the preview overlay's WindowTypeHint to Utility via
+# the Tauri gtk_window() escape hatch. Versions track what webkit2gtk 2.0
+# transitively depends on (GTK 3).
+gtk = "0.18"
+gdk = "0.18"

src-tauri/src/commands/windows.rs

@@ -66,6 +66,9 @@ pub async fn open_preview_window(app: tauri::AppHandle) -> Result<(), String> {
     // visible_on_all_workspaces sets _NET_WM_STATE_STICKY via GTK on X11
     // / XWayland so the overlay is pinned. Matches the ergonomic OpenWhispr
     // PR #183 shipped for KDE Plasma Wayland.
+    //
+    // Built hidden so we can set the Linux WindowTypeHint before the
+    // window maps — GTK3 only honours the hint pre-realize.
     let mut builder = WebviewWindowBuilder::new(
         &app,
         "transcription-preview",
@@ -79,6 +82,7 @@ pub async fn open_preview_window(app: tauri::AppHandle) -> Result<(), String> {
     .skip_taskbar(true)
     .visible_on_all_workspaces(true)
     .focused(false)
+    .visible(false)
     .decorations(use_native_decorations)
     .resizable(true);
 
@@ -88,7 +92,24 @@ pub async fn open_preview_window(app: tauri::AppHandle) -> Result<(), String> {
         }
     }
 
-    builder.build().map_err(|e| e.to_string())?;
+    let window = builder.build().map_err(|e| e.to_string())?;
+
+    // Defence-in-depth for non-KDE compositors (Hyprland, Sway, GNOME
+    // Mutter) where SKIP_TASKBAR alone may not hide a window from the
+    // alt-tab switcher. Classifying as Utility signals to the compositor
+    // that this is an assistive auxiliary window — switchers and tilers
+    // treat it accordingly. Noop on KWin (already handled by skip_taskbar)
+    // but harmless. Must happen before show() per GTK3 docs.
+    #[cfg(target_os = "linux")]
+    {
+        use gdk::WindowTypeHint;
+        use gtk::prelude::GtkWindowExt;
+        if let Ok(gtk_window) = window.gtk_window() {
+            gtk_window.set_type_hint(WindowTypeHint::Utility);
+        }
+    }
+
+    window.show().map_err(|e| e.to_string())?;
     Ok(())
 }