Reuse single toast slot for composer drop failures

Multiple drop failures now replace the previous toast instead of
stacking, preventing notification clutter. Uses a dedicated toast ID
separate from Source Control's stage/discard notifications.

src/renderer/src/hooks/composer-drop-failure-toast.test.ts

@@ -1,7 +1,7 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 const { toastError } = vi.hoisted(() => ({
-  toastError: vi.fn<(title: string, options?: { description?: string }) => void>()
+  toastError: vi.fn<(title: string, options?: { id?: string; description?: string }) => void>()
 }))
 vi.mock('sonner', () => ({ toast: { error: toastError } }))
 
@@ -15,10 +15,11 @@ const SKIP_REASON_COPY = [
   ['unsupported', 'Unsupported file type.']
 ] as const satisfies readonly (readonly [ImportSkipReason, string])[]
 
-function lastToast(): { title: string; description?: string } {
+function lastToast(): { title: string; id?: string; description?: string } {
   const call = toastError.mock.calls.at(-1)
   return {
     title: String(call?.[0]),
+    id: call?.[1]?.id,
     description: call?.[1]?.description
   }
 }
@@ -74,6 +75,14 @@ describe('showComposerDropFailureToast', () => {
     expect(lastToast().description).toBe('EACCES: permission denied')
   })
 
+  it('reuses one slot so a second failed drop replaces the first instead of stacking', () => {
+    showComposerDropFailureToast({ failureCount: 1, total: 2 })
+    const first = lastToast().id
+    showComposerDropFailureToast({ failureCount: 2, total: 3 })
+    expect(first).toBeDefined()
+    expect(lastToast().id).toBe(first)
+  })
+
   it('gives no reason at all when the batch failed for differing reasons', () => {
     showComposerDropFailureToast({ failureCount: 3, total: 6 })
     expect(lastToast().title).toBe('3 of 6 items could not be attached.')

src/renderer/src/hooks/composer-drop-failure-toast.ts

@@ -4,6 +4,9 @@ import { compactIpcErrorMessage } from '@/lib/ipc-error'
 import type { ComposerDropFailure } from './composer-drop-result'
 import type { ImportSkipReason } from '../../../shared/filesystem-import-result-types'
 
+// Own slot, not Source Control's: a drop failure must not erase an unread stage/discard failure.
+const DROP_FAILURE_TOAST_ID = 'composer-drop-failure'
+
 const SKIP_REASON_COPY: Record<ImportSkipReason, { key: string; fallback: string }> = {
   missing: {
     key: 'auto.hooks.useComposerState.attachSkipMissing',
@@ -46,6 +49,9 @@ export function showComposerDropFailureToast({
       '{{failureCount}} of {{count}} items could not be attached.',
       { failureCount, count: total }
     ),
-    { description: commonFailure ? failureDescription(commonFailure) : undefined }
+    {
+      id: DROP_FAILURE_TOAST_ID,
+      description: commonFailure ? failureDescription(commonFailure) : undefined
+    }
   )
 }