Replace custom AuthContext with react-oidc-context

Deletes the hand-rolled AuthContext/UserManager setup and replaces it
with AuthProvider from react-oidc-context. onSigninCallback clears the
OIDC code params from the URL (unless an error is present). TokenSync
bridges the library token into the existing api/client setTokenProvider
pattern. AuthGuard updated to use auth.isLoading/isAuthenticated/
signinRedirect from the library. CallbackPage simplified to a passive
error renderer — react-oidc-context processes the OIDC exchange itself.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.idea/.idea.Budget/.idea/indexLayout.xml

@@ -1,7 +1,9 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
   <component name="UserContentModel">
-    <attachedFolders />
+    <attachedFolders>
+      <Path>.plans</Path>
+    </attachedFolders>
     <explicitIncludes />
     <explicitExcludes />
   </component>