fix(a11y): add alt prop to PreviewSignature image element

Adds an alt prop with a translatable default ('Signature preview')
so consumers can pass context-specific descriptions while keeping
a sensible fallback for screen readers.

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

src/components/PreviewSignature/PreviewSignature.vue

@@ -8,6 +8,7 @@
 		<div v-show="isLoaded" class="wrapper">
 			<img v-show="isLoaded"
 				:src="imageData"
+				:alt="alt"
 				:style="{
 					width,
 					height,
@@ -41,6 +42,12 @@ export default {
 			required: false,
 			default: '',
 		},
+		alt: {
+			type: String,
+			required: false,
+			// TRANSLATORS Alt text for an image showing the user's handwritten, typed, or uploaded signature. Used as fallback when the parent component does not pass a more specific description, for example "Current signature" or "Confirm your initials".
+			default: () => t('libresign', 'Signature preview'),
+		},
 	},
 	data() {
 		return {