feat(v2): add agent tree, status bar, notifications, settings dialog (Phase 5)

Agent tree visualization (SVG) with horizontal layout and bezier edges.
Global status bar with pane counts, active agents pulse, token/cost totals.
Toast notification system with auto-dismiss and agent dispatcher integration.
Settings dialog with SQLite persistence for shell, cwd, and max panes.
Keyboard shortcuts: Ctrl+W close pane, Ctrl+, open settings.

v2/src/lib/stores/notifications.svelte.ts

@@ -0,0 +1,36 @@
+// Notification store — ephemeral toast messages
+
+export type NotificationType = 'info' | 'success' | 'warning' | 'error';
+
+export interface Notification {
+  id: string;
+  type: NotificationType;
+  message: string;
+  timestamp: number;
+}
+
+let notifications = $state<Notification[]>([]);
+
+const MAX_TOASTS = 5;
+const TOAST_DURATION_MS = 4000;
+
+export function getNotifications(): Notification[] {
+  return notifications;
+}
+
+export function notify(type: NotificationType, message: string): void {
+  const id = crypto.randomUUID();
+  notifications.push({ id, type, message, timestamp: Date.now() });
+
+  // Cap visible toasts
+  if (notifications.length > MAX_TOASTS) {
+    notifications = notifications.slice(-MAX_TOASTS);
+  }
+
+  // Auto-dismiss
+  setTimeout(() => dismissNotification(id), TOAST_DURATION_MS);
+}
+
+export function dismissNotification(id: string): void {
+  notifications = notifications.filter(n => n.id !== id);
+}