feat(frontend): add canvas agent empty state with server-driven starters

Render a CanvasAgentEmptyState in the Agent panel that fetches localized
starter content from the new agent-starters endpoint, with local defaults
as fallback. Clicking a starter focuses the prompt composer (new focus
handle). Includes accompanying UI polish: side-nav cleanup, animated
prompt examples, and canvas panel spacing tweaks.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/domain/design.ts

@@ -147,6 +147,19 @@ export type QuickAction = {
   icon: string;
 };
 
+export type AgentStarter = {
+  id: string;
+  label: string;
+  prompt: string;
+};
+
+export type AgentStarterConfig = {
+  title: string;
+  description: string;
+  previewImages: string[];
+  starters: AgentStarter[];
+};
+
 export type InspirationItem = {
   id: string;
   title: string;