feat: add toast notification system with success toasts

Adds ToastProvider/ToastViewport for in-app notifications with dedupe,
auto-dismiss, and action links. Wires success toasts to issue create,
issue update, and comment mutations. Adds live event toasts for activity,
agent status, and run status changes via LiveUpdatesProvider.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/Layout.tsx

@@ -9,6 +9,7 @@ import { NewProjectDialog } from "./NewProjectDialog";
 import { NewGoalDialog } from "./NewGoalDialog";
 import { NewAgentDialog } from "./NewAgentDialog";
 import { OnboardingWizard } from "./OnboardingWizard";
+import { ToastViewport } from "./ToastViewport";
 import { useDialog } from "../context/DialogContext";
 import { usePanel } from "../context/PanelContext";
 import { useCompany } from "../context/CompanyContext";
@@ -88,6 +89,7 @@ export function Layout() {
       <NewGoalDialog />
       <NewAgentDialog />
       <OnboardingWizard />
+      <ToastViewport />
     </div>
   );
 }