## Why

When testing and debugging the AI chat agent, the system prompt and model parameters (temperature, top-p, max tokens) are hardcoded or absent. Exposing these in the UI lets the developer iterate on prompt engineering without restarting the server, and makes the app useful as a prompt testing workbench.

## What Changes

### Phase 1: Expose system prompt and model parameters
- Add tabbed UI to the chat page: **Chat** tab (existing conversation), **System Prompt** tab (editable text area), **Model Settings** tab (temperature, top-p, max tokens sliders/inputs)
- Extend the API contract: `ChatRequest` gains optional `SystemPrompt` and `ModelSettings` fields
- Backend applies the system prompt as the first message in ChatHistory and passes model settings to execution settings

### Phase 2: Prompt templates with few-shot examples (future)
- System prompt becomes a template with placeholder variables
- UI for adding few-shot input/output example pairs
- Template engine generates the final system prompt from template + examples
- *Phase 2 is scoped but NOT implemented in this change*

## Capabilities

### New Capabilities
- `prompt-settings-ui`: Tabbed interface for system prompt editing and model parameter controls
- `prompt-settings-api`: API contract extensions for system prompt and model parameters

### Modified Capabilities
- `chat-ui`: Chat page changes from single-panel to tabbed layout
- `chat-streaming`: API accepts optional system prompt and model settings in the request

## Impact

- **ChatRequest.cs** (Shared): Add `SystemPrompt` and `ModelSettings` properties
- **New ModelSettings.cs** (Shared): Temperature, TopP, MaxTokens model
- **ChatController.cs** (API): Apply system prompt to ChatHistory, pass model settings to execution settings
- **Chat.razor** (Client): Wrap in MudTabs, add System Prompt and Model Settings tab panels
- **ChatApiClient.cs** (Client): Pass new fields in requests
- No new packages — MudTabs, MudTextField, MudSlider are all part of MudBlazor