feat(fp): WP-16 — component a11y: description wiring + alert role

Wires text-input's aria-describedby to the form-field description div
(the BSN hint was rendered but never announced), pins desc-before-error
ordering, and switches alert to role=alert for errors vs role=status
for info/ok/warning. Composition contract enforced by story play tests
(form-field+text-input, alert per variant) run in the WP-01 CI gate.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/auth/ui/login-form/login-form.component.ts

@@ -24,7 +24,13 @@ import { ButtonComponent } from '@shared/ui/button/button.component';
         i18n-description="@@login.bsnDescription"
         description="9 cijfers (demo: vul iets in)"
       >
-        <app-text-input inputId="bsn" [(ngModel)]="bsn" name="bsn" placeholder="123456789" />
+        <app-text-input
+          inputId="bsn"
+          hasDescription
+          [(ngModel)]="bsn"
+          name="bsn"
+          placeholder="123456789"
+        />
       </app-form-field>
 
       <app-form-field i18n-label="@@login.wachtwoordLabel" label="Wachtwoord" fieldId="pw" required>