Fix OIDC web flow redirect URI not being sent to IDP

The /login route was not passing an explicit redirect_uri to the IDP for
the web flow, so openid-client would silently pick a default which could
resolve to localhost:3001 if OIDC_REDIRECT_URI was not set.

- AuthSession.redirectUri is now required (non-optional)
- createAuthSession() requires a redirectUri; detects native vs web via
  the timetracker:// scheme prefix instead of presence/absence of the arg
- /login route resolves the URI explicitly: request param for native
  flows, config.oidc.redirectUri for web flows
- getAuthorizationUrl() reads redirect_uri from session, no longer
  accepts it as a separate argument
- handleCallback() uses session.redirectUri directly, removing the
  fallback to config.oidc.redirectUri

backend/src/auth/oidc.ts

@@ -41,11 +41,11 @@ export interface AuthSession {
   codeVerifier: string;
   state: string;
   nonce: string | undefined;
-  redirectUri?: string;
+  redirectUri: string;
 }
 
-export function createAuthSession(redirectUri?: string): AuthSession {
-  const isNative = !!redirectUri;
+export function createAuthSession(redirectUri: string): AuthSession {
+  const isNative = redirectUri.startsWith('timetracker://');
   return {
     codeVerifier: generators.codeVerifier(),
     state: generators.state(),
@@ -58,7 +58,7 @@ export function createAuthSession(redirectUri?: string): AuthSession {
   };
 }
 
-export function getAuthorizationUrl(session: AuthSession, redirectUri?: string): string {
+export function getAuthorizationUrl(session: AuthSession): string {
   const client = getOIDCClient();
   const codeChallenge = generators.codeChallenge(session.codeVerifier);
 
@@ -67,16 +67,13 @@ export function getAuthorizationUrl(session: AuthSession, redirectUri?: string):
     state: session.state,
     code_challenge: codeChallenge,
     code_challenge_method: 'S256',
+    redirect_uri: session.redirectUri,
   };
 
   if (session.nonce) {
     params.nonce = session.nonce;
   }
 
-  if (redirectUri) {
-    params.redirect_uri = redirectUri;
-  }
-  
   return client.authorizationUrl(params);
 }
 
@@ -86,8 +83,6 @@ export async function handleCallback(
 ): Promise<TokenSet> {
   const client = getOIDCClient();
 
-  const redirectUri = session.redirectUri || config.oidc.redirectUri;
-  
   const checks: Record<string, string | undefined> = {
     code_verifier: session.codeVerifier,
     state: session.state,
@@ -98,7 +93,7 @@ export async function handleCallback(
   }
 
   const tokenSet = await client.callback(
-    redirectUri,
+    session.redirectUri,
     params,
     checks,
   );

backend/src/routes/auth.routes.ts

@@ -9,6 +9,7 @@ import {
 } from "../auth/oidc";
 import { signBackendJwt } from "../auth/jwt";
 import { requireAuth, syncUser } from "../middleware/auth";
+import { config } from "../config";
 import type { AuthenticatedRequest } from "../types";
 import type { AuthSession } from "../auth/oidc";
 
@@ -49,11 +50,18 @@ router.get("/login", async (req, res) => {
   try {
     await ensureOIDC();
 
-    const redirectUri = req.query.redirect_uri as string | undefined;
-    console.log(`[auth/login] initiated (redirect_uri: ${redirectUri ?? '(web flow)'})`);
+    const isNativeFlow = !!req.query.redirect_uri;
+    // For the web flow no redirect_uri is supplied in the request — use the
+    // backend-configured value so the IDP always receives an explicit URI
+    // rather than relying on the openid-client library to pick a default.
+    const redirectUri = isNativeFlow
+      ? (req.query.redirect_uri as string)
+      : config.oidc.redirectUri;
+
+    console.log(`[auth/login] initiated (redirect_uri: ${redirectUri})`);
     const session = createAuthSession(redirectUri);
 
-    if (redirectUri) {
+    if (isNativeFlow) {
       // Native app flow: store session by state so /auth/token can retrieve it
       // without relying on the browser cookie jar.
       storeNativeSession(session);
@@ -63,7 +71,7 @@ router.get("/login", async (req, res) => {
       req.session.oidc = session;
     }
 
-    const authorizationUrl = getAuthorizationUrl(session, redirectUri);
+    const authorizationUrl = getAuthorizationUrl(session);
     console.log(`[auth/login] redirecting to IDP`);
     res.redirect(authorizationUrl);
   } catch (error) {