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"