fix(admin/system): address review concerns in ImportStatusCard

- Remove dead `message` field from both frontend ImportStatus types
  (field is now @JsonIgnore'd on the backend)
- Extract failure message ternary into `$derived` — business logic off
  the template (Felix)
- Add motion-reduce:animate-none to spinner — WCAG 2.1 SC 2.3.3 (Leonie)
- Replace text-green-600 with text-green-800 — WCAG AA contrast 6.1:1
  on bg-green-50 (Leonie)
- Add min-h-[44px] to all three buttons — WCAG 2.2 44px touch target (Leonie)
- Add 6 missing tests: IMPORT_FAILED_INTERNAL path, IDLE state text,
  null importStatus, ontrigger called on DONE/FAILED/IDLE buttons (Sara)

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

frontend/src/routes/admin/system/+page.svelte

@@ -11,7 +11,6 @@ let backfillHashesLoading = $state(false);
 type ImportStatus = {
 	state: 'IDLE' | 'RUNNING' | 'DONE' | 'FAILED';
 	statusCode: string;
-	message: string;
 	processed: number;
 	startedAt: string | null;
 };