fix:image url validation and signout post (#24420)

* refac(routers): reject external URLs in profile/model image handlers

* refac(ui): centralize image URL validation in safeImageUrl helper

* refac(auths): make signout POST-only

* refac: gate external profile image redirect behind ENABLE_PROFILE_IMAGE_URL_FORWARDING

Restore the 302 redirect for external http(s) profile image URLs in
the user and model profile-image endpoints, but gate it behind a new
ENABLE_PROFILE_IMAGE_URL_FORWARDING env flag (default: True).

Existing deployments that rely on external profile image forwarding
continue to work unchanged.  Operators who want to suppress the
redirect (to prevent client-side IP/UA/Referer leaks) can set the
flag to False.

src/lib/components/common/Image.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { WEBUI_BASE_URL } from '$lib/constants';
+	import { safeImageUrl } from '$lib/utils/safeImageUrl';
 
 	import { settings } from '$lib/stores';
 	import ImagePreview from './ImagePreview.svelte';
@@ -19,7 +20,7 @@
 	const i18n = getContext('i18n');
 
 	let _src = '';
-	$: _src = src.startsWith('/') ? `${WEBUI_BASE_URL}${src}` : src;
+	$: _src = safeImageUrl(src.startsWith('/') ? `${WEBUI_BASE_URL}${src}` : src);
 
 	let showImagePreview = false;
 </script>