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/apis/auths/index.ts

@@ -328,7 +328,7 @@ export const userSignOut = async () => {
 	let error = null;
 
 	const res = await fetch(`${WEBUI_API_BASE_URL}/auths/signout`, {
-		method: 'GET',
+		method: 'POST',
 		headers: {
 			'Content-Type': 'application/json'
 		},