feat: mouse back/forward navigation + history stack

- C patch (vendor/.../window.c): intercept GDK button 8/9 → dispatch
  CustomEvent('verstak:navigate-back'/'verstak:navigate-forward')
- App.svelte: navigation stack (snapshot-based history), alt+arrows,
  mouse button back/forward handlers, onNavigateBack/Forward
- WorkbenchHost: close via navigate-back event
- WorkspaceHost: workspace tab bar + tool panels
- wails-mock: full navigation, sidebar, vau...

.gitignore

@@ -5,3 +5,4 @@ build/bin/verstak-desktop
 smoke-platform
 plugins/
 vendor/
+\nfrontend/e2e-results/