feat: add basic chat interface with MudBlazor and propose responses API integration

Install MudBlazor 9.2.0, replace Bootstrap layout with MudLayout/MudAppBar,
create Chat.razor with message list, text input, auto-scroll, and hardcoded
responses. Add ChatMessage shared model. Remove template pages (Counter,
Weather), move health check to /health. Include OpenSpec change artifacts
for the upcoming wire-responses-api work.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openspec/changes/archive/2026-04-04-basic-chat-interface/proposal.md

@@ -0,0 +1,35 @@
+## 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