fix: Resolve build errors with border color and import paths

- Add missing --border CSS variable for light and dark mode
- Add border color to Tailwind config using hsl(var(--border))
- Fix import paths in sidebar.tsx (@/hooks/ instead of @/components/hooks/)
- Fix import path in toaster.tsx (@/hooks/ instead of @/components/hooks/)

These fixes ensure npm run dev / npm run build work out of the box.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/ui/toaster.tsx

@@ -1,6 +1,6 @@
 "use client"
 
-import { useToast } from "@/components/hooks/use-toast"
+import { useToast } from "@/hooks/use-toast"
 import {
   Toast,
   ToastClose,