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/ui/pages/BrandKitPage/index.css

@@ -649,16 +649,16 @@
 }
 
 .brand-kit-section-nav button {
-  min-height: 32px;
+  min-height: 40px;
   display: flex;
   align-items: center;
-  gap: 9px;
+  gap: 10px;
   padding: 0 8px;
   border-radius: 6px;
-  color: #676d77;
+  color: #5f6570;
   background: transparent;
-  font-size: 11px;
-  font-weight: 650;
+  font-size: 16px;
+  font-weight: 680;
   text-align: left;
 }
 
@@ -668,9 +668,9 @@
 }
 
 .brand-kit-section-nav button span {
-  width: 19px;
-  color: #a1a6ae;
-  font-size: 9px;
+  width: 22px;
+  color: #9aa0aa;
+  font-size: 14px;
   font-variant-numeric: tabular-nums;
 }