fix(l1): toast on intake failure in L1Dashboard

Final review flagged silent failure on intake error. Adds a toast with
the backend detail message (or fallback) on catch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/l1/L1Dashboard.tsx

@@ -3,6 +3,7 @@ import { useNavigate } from 'react-router-dom'
 import { PageMeta } from '@/components/common/PageMeta'
 import { useAuthStore } from '@/store/authStore'
 import { l1Api } from '@/api/l1'
+import { toast } from '@/lib/toast'
 import { EmptyStateCard } from '@/components/l1/EmptyStateCard'
 import { ResumeInProgress } from '@/components/l1/ResumeInProgress'
 import type { QueueRow } from '@/types/l1'
@@ -46,6 +47,11 @@ export default function L1Dashboard() {
         customer_contact: customerContact.trim() || undefined,
       })
       navigate(`/l1/walk/${response.session_id}`)
+    } catch (err) {
+      const detail = (err as any)?.response?.data?.detail
+      const msg =
+        typeof detail === 'string' ? detail : 'Failed to start walk. Try again.'
+      toast.error(msg)
     } finally {
       setSubmitting(false)
     }