replace alerts with toasts

src/app/page.tsx

@@ -14,6 +14,7 @@ import { parseICS, generateICS } from '@/lib/ical'
 import type { CalendarEvent } from '@/lib/types'
 import { Textarea } from '@/components/ui/textarea'
 import { useSession } from 'next-auth/react'
+import { toast } from 'sonner'
 
 export default function HomePage() {
   const [events, setEvents] = useState<CalendarEvent[]>([])
@@ -128,7 +129,7 @@ export default function HomePage() {
       if (file.name.endsWith('.ics')) {
         handleImport(file)
       } else {
-        alert('Please drop an .ics file')
+        toast.warning('Please drop an .ics file')
       }
     }
   }
@@ -145,7 +146,7 @@ export default function HomePage() {
       })
 
       if (res.status === 401) {
-        alert('Please sign in to use AI features.')
+        toast.info('Please sign in to use AI features.')
         setAiLoading(false)
         return
       }
@@ -185,11 +186,11 @@ export default function HomePage() {
           setSummaryUpdated(new Date().toLocaleString())
         }
       } else {
-        alert('AI did not return event data.')
+        toast.error('AI did not return event data.')
       }
     } catch (err) {
       console.error(err)
-      alert('Error from AI service.')
+      toast.error('Error from AI service.')
     } finally {
       setAiLoading(false)
     }