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/specs/mudblazor-setup/spec.md

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+## ADDED Requirements
+
+### Requirement: MudBlazor package installed
+
+The Client project SHALL have MudBlazor 9.2.0 installed as a NuGet dependency.
+
+#### Scenario: Package reference present
+
+- **WHEN** the Client project is built
+- **THEN** MudBlazor 9.2.0 is resolved as a dependency
+
+### Requirement: MudBlazor services registered
+
+MudBlazor services SHALL be registered in the Client's DI container via `AddMudServices()`.
+
+#### Scenario: Services available
+
+- **WHEN** the application starts
+- **THEN** MudBlazor services (snackbar, dialog, etc.) are available for injection
+
+### Requirement: MudBlazor assets loaded
+
+The Client's `index.html` SHALL include MudBlazor CSS, JS, and font references.
+
+#### Scenario: Styles and scripts present
+
+- **WHEN** the application loads in the browser
+- **THEN** MudBlazor CSS (`_content/MudBlazor/MudBlazor.min.css`), JS (`_content/MudBlazor/MudBlazor.min.js`), and Material Design Icons font are loaded
+
+### Requirement: MudBlazor layout providers
+
+The app root SHALL include `MudThemeProvider`, `MudPopoverProvider`, and `MudDialogProvider` so MudBlazor components function correctly.
+
+#### Scenario: Providers present
+
+- **WHEN** any MudBlazor component is rendered
+- **THEN** it functions correctly because the required providers are in the component tree
+
+### Requirement: MudBlazor layout replaces Bootstrap
+
+The application layout SHALL use MudBlazor layout components (`MudLayout`, `MudAppBar`, `MudMainContent`) instead of the current Bootstrap navbar.
+
+#### Scenario: Layout renders with MudBlazor
+
+- **WHEN** any page is displayed
+- **THEN** the page is wrapped in a MudBlazor layout with an app bar showing the application name