fix(import): address round-3 review concerns

- Add comment to openFileStream() explaining package-private visibility
  is intentional (Mockito spy seam for IOException test)
- Key {#each} skippedFiles by filename instead of array index
- Add test: skipped section hidden when state is FAILED
- Add test: reasonLabel returns raw code for unknown reason strings

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

frontend/src/routes/admin/system/ImportStatusCard.svelte

@@ -78,7 +78,7 @@ function reasonLabel(code: string): string {
 					</div>
 				</summary>
 				<ul class="mt-3 space-y-1">
-					{#each importStatus.skippedFiles as skipped, i (i)}
+					{#each importStatus.skippedFiles as skipped (skipped.filename)}
 						<li class="font-mono text-sm text-ink-2">
 							{skipped.filename} — {reasonLabel(skipped.reason)}
 						</li>