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/specs/email-upload/spec.md

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+## Purpose
+
+Define the email upload UX — drag-and-drop, file picker, and upload behavior constraints during streaming.
+
+## Requirements
+
+### Requirement: Drag-and-drop email upload
+
+The chat message area SHALL accept files dragged from the desktop or file explorer. When a supported file is dropped, the client SHALL read the file content and send it to the extraction endpoint.
+
+#### Scenario: Drag HTML file onto chat
+
+- **WHEN** the user drags an .html file over the message area
+- **THEN** a visual drop indicator appears (e.g., highlighted border, overlay text "Drop email here")
+
+#### Scenario: Drop HTML file triggers extraction
+
+- **WHEN** the user drops an .html file onto the message area
+- **THEN** the client reads the HTML content, sends it to `POST /api/chat/extract`, and streams the extraction response in the chat
+
+#### Scenario: Unsupported file type rejected
+
+- **WHEN** the user drops a non-.html file (e.g., .pdf, .docx)
+- **THEN** the client shows a brief error message indicating only .html files are supported
+
+### Requirement: File picker upload button
+
+The chat input area SHALL include an upload button (e.g., attachment icon) that opens a file picker dialog for selecting .html email files.
+
+#### Scenario: Upload via file picker
+
+- **WHEN** the user clicks the upload button and selects an .html file
+- **THEN** the client reads the HTML content and sends it to the extraction endpoint, same as drag-and-drop
+
+### Requirement: Upload disabled during streaming
+
+The upload zone and file picker SHALL be disabled while a response is streaming.
+
+#### Scenario: Drop during streaming
+
+- **WHEN** the user attempts to drop a file while the assistant is streaming
+- **THEN** the drop is ignored and no extraction request is sent