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...

internal/api/app.go

@@ -113,6 +113,7 @@ func (a *App) ensureWorkbench() *coreworkbench.Router {
 func (a *App) Startup(ctx context.Context) {
 	a.ctx = ctx
 	log.Printf("[api] App.Startup: initialized with %d plugins", len(a.plugins))
+	startMouseMonitor(ctx)
 }
 
 func (a *App) findPlugin(pluginID string) (*plugin.Plugin, error) {