Add shadcn sidebar/breadcrumb/collapsible/avatar/tooltip (base-nova)

Registry-installed Base UI flavor; existing ui components untouched.
Also drops the meaningless empty registries key from components.json.

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

frontend/components.json

@@ -20,6 +20,5 @@
     "hooks": "@/hooks"
   },
   "menuColor": "default-translucent",
-  "menuAccent": "subtle",
-  "registries": {}
+  "menuAccent": "subtle"
 }