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/wire-responses-api/specs/chat-ui/spec.md

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+## MODIFIED Requirements
+
+### Requirement: Hardcoded response
+
+The assistant SHALL reply with a real AI response streamed from the backend API, replacing the previous hardcoded stub. Tokens appear incrementally as they arrive.
+
+#### Scenario: Bot replies with streamed AI response
+
+- **WHEN** the user sends any message
+- **THEN** the assistant message appears and grows token by token as the stream delivers text
+
+### Requirement: Message input
+
+The chat page SHALL provide a text input area at the bottom of the page where the user can type and submit messages.
+
+#### Scenario: Submit via button
+
+- **WHEN** the user types text and clicks the send button
+- **THEN** the message is added to the conversation and the input is cleared
+
+#### Scenario: Submit via Enter key
+
+- **WHEN** the user types text and presses Enter
+- **THEN** the message is submitted (same as clicking send)
+
+#### Scenario: Empty input blocked
+
+- **WHEN** the user attempts to send an empty or whitespace-only message
+- **THEN** nothing is sent and no message is added
+
+#### Scenario: Input disabled during streaming
+
+- **WHEN** the assistant is currently streaming a response
+- **THEN** the input field and send button are disabled until streaming completes
+
+## ADDED Requirements
+
+### Requirement: Thinking indicator
+
+The chat page SHALL show a visual indicator while waiting for the first token from the assistant.
+
+#### Scenario: Indicator shown during wait
+
+- **WHEN** the user sends a message and the assistant has not yet started streaming
+- **THEN** a thinking indicator (e.g., animated dots) is shown in the assistant message area
+
+#### Scenario: Indicator replaced by content
+
+- **WHEN** the first token arrives from the stream
+- **THEN** the thinking indicator is replaced by the streamed text