Fix asset paths for gemini branch

[johnpryan]

This fixes some issues that I found when deploying a preview version of the app to Firebase Hosting:

• adjusts codemirror path to work in different Firebase Hosting scenarios
• adjusts the asset configuration to make sure the Gemini logo loads when deployed to Firebase Hosting.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/fibonacci.dart
pkgs/samples/lib/google_ai.dart
pkgs/samples/lib/hello_world.dart
pkgs/samples/lib/main.dart
pkgs/samples/lib/sunflower.dart