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/tasks.md

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+## 1. MudBlazor Setup
+
+- [x] 1.1 Install MudBlazor 9.2.0 NuGet package in ChatAgent.Client
+- [x] 1.2 Add MudBlazor CSS, JS, and Material Design Icons font to index.html (remove Bootstrap CSS)
+- [x] 1.3 Add `@using MudBlazor` to _Imports.razor
+- [x] 1.4 Register MudBlazor services (`AddMudServices()`) in Program.cs
+- [x] 1.5 Add MudThemeProvider, MudPopoverProvider, MudDialogProvider to MainLayout.razor
+
+## 2. Layout Migration
+
+- [x] 2.1 Replace MainLayout.razor with MudBlazor layout (MudLayout, MudAppBar, MudMainContent)
+- [x] 2.2 Remove NavMenu.razor (Bootstrap navbar no longer needed)
+- [x] 2.3 Remove MainLayout.razor.css (MudBlazor handles styling)
+
+## 3. Shared Model
+
+- [x] 3.1 Create ChatMessage.cs in ChatAgent.Shared/Models with Role, Content, Timestamp
+
+## 4. Chat Page
+
+- [x] 4.1 Create Chat.razor at route `/` with message list and input area
+- [x] 4.2 Implement message display with MudPaper cards (distinct styling for user vs assistant)
+- [x] 4.3 Implement text input with MudTextField and send button adornment
+- [x] 4.4 Wire Enter key and send button to submit handler
+- [x] 4.5 Block empty/whitespace-only submissions
+- [x] 4.6 Add hardcoded assistant response after each user message
+- [x] 4.7 Implement auto-scroll to bottom on new messages
+
+## 5. Cleanup
+
+- [x] 5.1 Move Home.razor route from `/` to `/health`
+- [x] 5.2 Remove Counter.razor and Weather.razor
+- [x] 5.3 Update app.css — remove Bootstrap-specific styles, keep custom styles that still apply
+
+## 6. Verify
+
+- [x] 6.1 Run `dotnet build` on the solution to confirm no errors
+- [ ] 6.2 Manually verify: chat page loads at `/`, messages display correctly, hardcoded response works