Display an error message when OAuth authentication fails

Currently, when ADK Web directs a user through an OAuth authentication flow and the flow fails, no error message or guidance is shown to the user. This results in a silent failure where the user is redirected back to ADK Web without any indication of what went wrong or how to resolve it.

I suggest displaying a notification banner at the bottom of the screen when the user is redirected back with an error query parameter in the URL.

**Steps to Reproduce**
* Create an invalid OAuth configuration by requesting an invalid scope.
* Create an agent that accesses an MCP resource protected by OAuth.
* Run ADK Web.
* Ask the agent a question to trigger the authentication flow.
* The OAuth flow redirects back to ADK Web with an error query parameter.

**Expected Behavior**
ADK Web should display a clear, user-facing error message indicating that the OAuth authentication failed and the reason.

**Actual Behavior**
The user is redirected back silently with no visual feedback or error message shown, making it unclear that the authentication failed.

**Additional Context**
This behavior breaks the user flow and can be confusing, especially for first-time users setting up OAuth-protected MCP resources.