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/chat-ui/spec.md

@@ -0,0 +1,15 @@
+## MODIFIED Requirements
+
+### Requirement: Chat page is default route
+
+The chat page SHALL be routed at `/sales-assistant` (or `/` with redirect). The page content SHALL be wrapped in a MudTabs container with the conversation UI in the first tab panel.
+
+#### Scenario: Page loads with chat tab active
+
+- **WHEN** the user navigates to the chat page
+- **THEN** the Chat tab is active showing the message list and input area
+
+#### Scenario: Chat functionality unchanged
+
+- **WHEN** the user sends a message from the Chat tab
+- **THEN** the assistant response streams in exactly as before, with the same SSE contract and rendering behavior