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/utils/safeImageUrl.ts

@@ -0,0 +1,33 @@
+import { WEBUI_BASE_URL } from '$lib/constants';
+
+const PLACEHOLDER_IMAGE = '/favicon.png';
+
+/**
+ * Validates an image URL against an allowlist of safe patterns and returns
+ * the URL if trusted, or a placeholder otherwise.
+ *
+ * Allowed patterns:
+ *   - Relative paths (starting with '/')
+ *   - data:image/* URIs
+ *   - Same-origin URLs (starting with WEBUI_BASE_URL)
+ *   - Gravatar URLs (https://www.gravatar.com/avatar/)
+ *
+ * All other URLs (including arbitrary http(s):// origins) are rejected to
+ * prevent client-side IP/UA/Referer leaks to attacker-controlled servers.
+ */
+export function safeImageUrl(url: string): string {
+	if (!url || url === '') {
+		return `${WEBUI_BASE_URL}${PLACEHOLDER_IMAGE}`;
+	}
+
+	if (
+		url.startsWith(WEBUI_BASE_URL) ||
+		url.startsWith('https://www.gravatar.com/avatar/') ||
+		url.startsWith('data:') ||
+		url.startsWith('/')
+	) {
+		return url;
+	}
+
+	return `${WEBUI_BASE_URL}${PLACEHOLDER_IMAGE}`;
+}