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/i18n/locales/en-US.json

@@ -452,6 +452,14 @@
   "agentDesign": "Agent Design",
   "agentShort": "Agent",
   "agentDesignHint": "Use web search and image models to advance this canvas.",
+  "agentEmptyDescription": "Enter a prompt to begin, or use one of the starting points below.",
+  "agentEmptyStarterLabel": "Creative starting points",
+  "agentEmptyTitle": "What would you like to create today?",
+  "agentStarterBrandVisual": "Brand visual",
+  "agentStarterEcommerce": "E-commerce product",
+  "agentStarterLogo": "Logo design",
+  "agentStarterPoster": "Poster",
+  "agentStarterSocialMedia": "Social media",
   "agentWebSearch": "Web research",
   "agentWebSearchHint": "Search the web and summarize usable design evidence.",
   "referenceLibrary": "Reference library",