feat: add extraction schema, sidebar nav, few-shot prompting, and prompt settings

Overhaul extraction pipeline with new TradeItem model, conversation flow,
and dedicated extraction endpoint. Add sidebar navigation with NavMenu
component and landing page. Introduce few-shot prompting service and
tests. Add prompt settings and email upload specs. Update OpenSpec
tooling with improved export-spec and extract-feature commands. Archive
completed changes and export full specs.

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

openspec/changes/archive/2026-04-06-expose-prompt-settings/specs/prompt-settings-api/spec.md

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+## ADDED Requirements
+
+### Requirement: ModelSettings shared model
+
+The Shared project SHALL define a `ModelSettings` class with nullable properties: `Temperature` (double?), `TopP` (double?), `MaxTokens` (int?). Null values indicate "use server default".
+
+#### Scenario: All fields null
+
+- **WHEN** a ModelSettings instance has all null fields
+- **THEN** the backend uses Semantic Kernel default values for all parameters
+
+#### Scenario: Partial override
+
+- **WHEN** a ModelSettings instance has Temperature set but TopP and MaxTokens null
+- **THEN** only Temperature is overridden; other parameters use defaults
+
+### Requirement: System prompt in chat request
+
+The `ChatRequest` SHALL accept an optional `SystemPrompt` (string?) property. When present and non-empty, the backend SHALL insert it as the first system message in the ChatHistory before user/assistant messages.
+
+#### Scenario: System prompt provided
+
+- **WHEN** a ChatRequest includes a non-empty SystemPrompt
+- **THEN** the ChatHistory starts with a system message containing that text, followed by the conversation messages
+
+#### Scenario: System prompt absent
+
+- **WHEN** a ChatRequest has a null or empty SystemPrompt
+- **THEN** the ChatHistory contains only user and assistant messages (no system message)
+
+### Requirement: Model settings in chat request
+
+The `ChatRequest` SHALL accept an optional `Settings` (ModelSettings?) property. When present, the backend SHALL apply non-null values to `OpenAIPromptExecutionSettings` before calling the Semantic Kernel.
+
+#### Scenario: Temperature override
+
+- **WHEN** a ChatRequest includes Settings with Temperature = 0.5
+- **THEN** the OpenAIPromptExecutionSettings.Temperature is set to 0.5
+
+#### Scenario: No settings provided
+
+- **WHEN** a ChatRequest has null Settings
+- **THEN** the backend uses default OpenAIPromptExecutionSettings (only FunctionChoiceBehavior.Auto is set)