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-email-upload-ux/specs/chat-ui/spec.md

@@ -0,0 +1,30 @@
+## MODIFIED Requirements
+
+### 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. The input area SHALL also include a file upload button for triggering email extraction.
+
+#### 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, send button, and upload button are disabled until streaming completes
+
+#### Scenario: Upload button opens file picker
+
+- **WHEN** the user clicks the upload button in the input area
+- **THEN** a file picker dialog opens filtered to .html files