feat: intercept GDK button 8/9 for mouse back/forward navigation

WebKitGTK does not propagate XButton1/XButton2 (buttons 8 and 9) into
DOM events — event.button and event.buttons are always 0 for these
clicks. This prevents the frontend from detecting hardware back/forward
mouse buttons for history navigation.

Solution: patch Wails' window.c on Linux to intercept button-press-event
at the GTK signal level (before WebKit processes it). For button 8/9 we
call webkit_web_view_run_javascript() to dispatch a native CustomEvent
('verstak:navigate-back' / 'verstak:navigate-forward') into the page,
allowing the frontend to navigate history without any workaround on the
JS side.

The patch is applied automatically during build via scripts/build.sh:
  go mod vendor → patch -p0 < patches/window.c.button-press.patch
Vendor directory is gitignored.

.gitignore

@@ -4,3 +4,4 @@ frontend/dist/
 build/bin/verstak-desktop
 smoke-platform
 plugins/
+vendor/