Refactor session management and enhance OAuth2 callback handling

- Updated session management functions to include cookie handling for session creation and destruction.
- Introduced applySessionCookie and clearSessionCookie functions for better cookie management in responses.
- Enhanced error logging in the OAuth2 callback to improve debugging.
- Revised PHASE-TRACKING.md to reflect changes in session handling and OAuth2 integration.

apps/webui/src/app/api/auth/callback/route.ts

@@ -3,7 +3,7 @@ import { type NextRequest, NextResponse } from 'next/server';
 import { exchangeCodeForToken, fetchDiscordUser } from '@/lib/discord-oauth';
 import { env } from '@/lib/env';
 import { redis } from '@/lib/redis';
-import { createSession } from '@/lib/session';
+import { applySessionCookie, createSession } from '@/lib/session';
 
 function oauthStateKey(state: string): string {
   return `webui:oauth:state:${state}`;
@@ -36,6 +36,7 @@ export async function GET(request: NextRequest) {
   }
   await redis.del(stateKey);
 
+  let cookieValue: string;
   try {
     const token = await exchangeCodeForToken(code);
     const discordUser = await fetchDiscordUser(token.access_token);
@@ -47,14 +48,19 @@ export async function GET(request: NextRequest) {
       avatar: discordUser.avatar ?? null
     };
 
-    await createSession({
+    cookieValue = await createSession({
       user,
       accessToken: token.access_token,
       tokenExpiresAt: Date.now() + token.expires_in * 1000
     });
-  } catch {
+  } catch (error) {
+    console.error('OAuth callback failed', error);
     return loginRedirect('oauth_failed');
   }
 
-  return NextResponse.redirect(new URL('/dashboard', env.WEBUI_URL));
+  // Cookie must be set on the redirect response — cookies().set() alone is not
+  // reliably attached when returning NextResponse.redirect() from a route handler.
+  const response = NextResponse.redirect(new URL('/dashboard', env.WEBUI_URL));
+  applySessionCookie(response, cookieValue);
+  return response;
 }