## Why

The project exists to be a working AI chat interface, but there is no chat UI yet — only a health check page. This change builds the foundational chat experience using MudBlazor components, with hardcoded responses so the UI can be developed and tested independently of the OpenAI API integration.

## What Changes

- Install and configure MudBlazor in the Client project (NuGet, CSS/JS, providers, imports)
- Replace the Bootstrap navbar/layout with a MudBlazor layout (MudLayout, MudAppBar, MudMainContent)
- Create a Chat page with a message list and text input, styled after ChatGPT/Gemini
- Add a shared `ChatMessage` model (role + content + timestamp)
- Wire the input to append user messages and reply with a hardcoded bot response
- Remove template pages (Counter, Weather) that are no longer needed

## Capabilities

### New Capabilities
- `chat-ui`: The chat interface — message display, input handling, auto-scroll, and layout
- `mudblazor-setup`: MudBlazor installation, theming, and provider configuration

### Modified Capabilities
<!-- None -->

## Impact

- `src/ChatAgent.Client/ChatAgent.Client.csproj`: Add MudBlazor package
- `src/ChatAgent.Client/wwwroot/index.html`: Add MudBlazor CSS/JS/font links
- `src/ChatAgent.Client/_Imports.razor`: Add MudBlazor using
- `src/ChatAgent.Client/Program.cs`: Add MudBlazor services
- `src/ChatAgent.Client/Layout/MainLayout.razor`: Replace with MudBlazor layout
- `src/ChatAgent.Client/Layout/NavMenu.razor`: Replace or remove Bootstrap nav
- `src/ChatAgent.Client/Pages/Chat.razor`: New chat page (becomes default route)
- `src/ChatAgent.Client/Pages/Home.razor`: Demote from `/` route or keep as `/health`
- `src/ChatAgent.Shared/Models/ChatMessage.cs`: New shared message model
- `src/ChatAgent.Client/Pages/Counter.razor`: Remove
- `src/ChatAgent.Client/Pages/Weather.razor`: Remove