FIX: Correct OIDC cookie name mismatch across all auth endpoints

**Root Cause:**
- Auth system was looking for 'keycloak-session' cookies
- But actual OIDC system uses 'nuxt-oidc-auth' cookies
- This caused authentication failures for file previews and other endpoints

**Files Updated:**
- server/utils/auth.ts: Updated to check 'nuxt-oidc-auth' cookie
- server/api/auth/session.ts: Updated cookie name references
- server/api/auth/logout.ts: Updated cookie deletion
- server/api/auth/keycloak/callback.ts: Updated cookie creation

**Result:**
- File previews should now work for authenticated users
- All authentication endpoints now use consistent cookie names
- Both x-tag headers and OIDC sessions work correctly

server/api/auth/keycloak/callback.ts

@@ -60,14 +60,14 @@ export default defineEventHandler(async (event) => {
     }
 
     // Create a simple session using a secure cookie
-    setCookie(event, 'keycloak-session', JSON.stringify(sessionData), {
+    setCookie(event, 'nuxt-oidc-auth', JSON.stringify(sessionData), {
       httpOnly: true,
       secure: true,
       sameSite: 'lax',
       maxAge: tokenResponse.expires_in
     })
 
-    console.log('[KEYCLOAK] Session cookie set, redirecting to dashboard')
+    console.log('[OIDC] Session cookie set, redirecting to dashboard')
 
     // Redirect to dashboard
     await sendRedirect(event, '/dashboard')