add: mandate_date_signed display

frontend/src/routes/auth/about/[id]/+page.svelte

@@ -607,6 +607,13 @@
           bind:value={user.bank_account.mandate_reference}
           placeholder={$t("placeholder.mandate_reference")}
         />
+        <InputField
+          name="mandate_date_signed"
+          label={$t("mandate_date_signed")}
+          type="date"
+          bind:value={user.bank_account.mandate_date_signed}
+          readonly={true}
+        />
       </div>
       <div class="button-container">
         {#if isUpdating}