refactor: switch auth data from localStorage to sessionStorage

Aligns user and permissions storage with the existing sessionStorage-based
token model. Eliminates the dual-write pattern and stale cross-session data.

webui/static/pages/passkeys.js

@@ -92,7 +92,7 @@ function addCredentialModal() {
                         setModalProcessing(true);
                         refreshModals();
 
-                        const user = JSON.parse(localStorage.getItem('vw:user') || 'null');
+                        const user = JSON.parse(sessionStorage.getItem('vw:user') || 'null');
                         const username = user?.username || '';
                         if (!username) {
                             toast('Username not available', 'error');

webui/static/pages/users.js

@@ -21,12 +21,12 @@ const SUBSYSTEMS = [
 ];
 
 function currentUser() {
-    const u = JSON.parse(localStorage.getItem('vw:user') || 'null');
+    const u = JSON.parse(sessionStorage.getItem('vw:user') || 'null');
     return u ? u.username : '';
 }
 
 function hasAuthAdmin() {
-    const perms = JSON.parse(localStorage.getItem('vw:permissions') || 'null');
+    const perms = JSON.parse(sessionStorage.getItem('vw:permissions') || 'null');
     return perms && perms.auth === 'rw';
 }