fix(frontend): announce blocking validation errors via role=alert

ClaimDrawerError.tsx already wraps its error surface with role=alert
(line 36). Apply the same pattern to the ValidationPanel errors
sub-section so assistive tech announces blocking validation failures
when the drawer opens, matching the established sibling pattern.

Warnings stay decorative (no role change) — they're advisories, not
blocks.

src/components/ClaimDrawer/ValidationPanel.tsx

@@ -129,6 +129,7 @@ export function ValidationPanel({ validation }: ValidationPanelProps) {
         <div
           data-testid="validation-errors"
           data-rule-group="errors"
+          role="alert"
           className="flex flex-col gap-2 border-l-2 border-[color:var(--m-error)] bg-[color:var(--m-error-bg)] px-3 py-2.5"
         >
           <div className="flex items-center gap-2">