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/chat/Messages/ProfileImage.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { WEBUI_BASE_URL } from '$lib/constants';
+	import { safeImageUrl } from '$lib/utils/safeImageUrl';
 
 	export let className = 'size-8';
 	export let src = `${WEBUI_BASE_URL}/static/favicon.png`;
@@ -7,14 +8,7 @@
 
 <img
 	aria-hidden="true"
-	src={src === ''
-		? `${WEBUI_BASE_URL}/static/favicon.png`
-		: src.startsWith(WEBUI_BASE_URL) ||
-			  src.startsWith('https://www.gravatar.com/avatar/') ||
-			  src.startsWith('data:') ||
-			  src.startsWith('/')
-			? src
-			: `${WEBUI_BASE_URL}/user.png`}
+	src={safeImageUrl(src)}
 	class=" {className} object-cover rounded-full"
 	alt="profile"
 	draggable="false"