Add duplicate pattern check when adding budget rules

Client side catches it immediately (case-insensitive match against
existing rules) and shows an inline error. API also rejects with 409
as the authoritative guard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/api/budget-rules/route.ts

@@ -29,6 +29,11 @@ export async function POST(req: Request) {
   const budget = await prisma.budget.findFirst({ where: { id: budgetId, userId: session.user.id } })
   if (!budget) return NextResponse.json({ error: 'Budget not found' }, { status: 404 })
 
+  const existing = await prisma.budgetRule.findFirst({
+    where: { budgetId, pattern: { equals: pattern, mode: 'insensitive' } },
+  })
+  if (existing) return NextResponse.json({ error: 'Duplicate rule pattern' }, { status: 409 })
+
   const rule = await prisma.budgetRule.create({
     data: { userId: session.user.id, budgetId, pattern },
     select: { id: true, budgetId: true, pattern: true },

src/components/budgets/BudgetRulesDialog.tsx

@@ -24,10 +24,14 @@ export function BudgetRulesDialog({ open, onOpenChange, budgetId, budgetName, ru
   const router = useRouter()
   const [pattern, setPattern] = useState('')
   const [adding, setAdding] = useState(false)
+  const [dupError, setDupError] = useState(false)
 
   async function handleAdd(e: React.FormEvent) {
     e.preventDefault()
     if (!pattern.trim()) return
+    const isDup = rules.some((r) => r.pattern.toLowerCase() === pattern.trim().toLowerCase())
+    if (isDup) { setDupError(true); return }
+    setDupError(false)
     setAdding(true)
     await fetch('/api/budget-rules', {
       method: 'POST',
@@ -36,6 +40,7 @@ export function BudgetRulesDialog({ open, onOpenChange, budgetId, budgetName, ru
     })
     setPattern('')
     setAdding(false)
+    setDupError(false)
     router.refresh()
   }
 
@@ -78,17 +83,22 @@ export function BudgetRulesDialog({ open, onOpenChange, budgetId, budgetName, ru
           </ul>
         )}
 
-        <form onSubmit={handleAdd} className="flex gap-2">
+        <form onSubmit={handleAdd} className="space-y-1.5">
+          <div className="flex gap-2">
             <Input
               placeholder="e.g. Amazon, Netflix, Starbucks"
               value={pattern}
-            onChange={(e) => setPattern(e.target.value)}
-            className="flex-1"
+              onChange={(e) => { setPattern(e.target.value); setDupError(false) }}
+              className={dupError ? 'flex-1 border-destructive focus-visible:ring-destructive' : 'flex-1'}
             />
             <Button type="submit" disabled={adding || !pattern.trim()} size="sm">
               <Plus className="h-4 w-4 mr-1" />
               Add
             </Button>
+          </div>
+          {dupError && (
+            <p className="text-xs text-destructive">That pattern already exists for this budget.</p>
+          )}
         </form>
       </DialogContent>
     </Dialog>