Introduce canvasPixelTest() intended to validate canvas functionality

Browsers and plugins that spoof canvas data produce corrupt images. In attempt to mitigate:

* Add canvasPixelTest() to test a single pixel and test the RGB values
* Test canvasPixelTest() inside generateInitialsImage() and use default `/user.png` if failure detected
* Call canvasPixelTest() directly within settings to avoid setting an invalid image
* Use toast.error() with 10 second autoClose

src/routes/auth/+page.svelte

@@ -5,7 +5,7 @@
 	import { WEBUI_NAME, config, user } from '$lib/stores';
 	import { onMount, getContext } from 'svelte';
 	import { toast } from 'svelte-sonner';
-	import { generateInitialsImage } from '$lib/utils';
+	import { generateInitialsImage, canvasPixelTest } from '$lib/utils';
 
 	const i18n = getContext('i18n');
 
@@ -43,6 +43,12 @@
 			}
 		);
 
+		if (!canvasPixelTest()) {
+			toast.error("Canvas pixel test failed, fingerprint evasion likely. Default image used.", {
+				autoClose: 1000 * 10,
+			});
+		}
+
 		await setSessionUser(sessionUser);
 	};