docs: add LEM Lab conversational training pipeline design

Design doc for LEM's chat-driven training pipeline covering
prompt-response capture, DPO pair generation, and LoRA fine-tuning
flow for local MLX models.

Co-Authored-By: Virgil <virgil@lethean.io>

docs/plans/2026-02-17-lem-chat-design.md

@@ -0,0 +1,82 @@
+# LEM Chat — Web Components Design
+
+**Date**: 2026-02-17
+**Status**: Approved
+
+## Summary
+
+Standalone chat UI built with vanilla Web Components (Custom Elements + Shadow DOM). Connects to the MLX inference server's OpenAI-compatible SSE streaming endpoint. Zero framework dependencies. Single JS file output, embeddable anywhere.
+
+## Components
+
+| Element | Purpose |
+|---------|---------|
+| `<lem-chat>` | Container. Conversation state, SSE connection, config via attributes |
+| `<lem-messages>` | Scrollable message list with auto-scroll anchoring |
+| `<lem-message>` | Single message bubble. Streams tokens for assistant messages |
+| `<lem-input>` | Text input, Enter to send, Shift+Enter for newline |
+
+## Data Flow
+
+```
+User types in <lem-input>
+  → dispatches 'lem-send' CustomEvent
+  → <lem-chat> catches it
+  → adds user message to <lem-messages>
+  → POST /v1/chat/completions {stream: true, messages: [...history]}
+  → reads SSE chunks via fetch + ReadableStream
+  → appends tokens to streaming <lem-message>
+  → on [DONE], finalises message
+```
+
+## Configuration
+
+```html
+<lem-chat endpoint="http://localhost:8090" model="qwen3-8b"></lem-chat>
+```
+
+Attributes: `endpoint`, `model`, `system-prompt`, `max-tokens`, `temperature`
+
+## Theming
+
+Shadow DOM with CSS custom properties:
+
+```css
+--lem-bg: #1a1a1e;
+--lem-msg-user: #2a2a3e;
+--lem-msg-assistant: #1e1e2a;
+--lem-accent: #5865f2;
+--lem-text: #e0e0e0;
+--lem-font: system-ui;
+```
+
+## Markdown
+
+Minimal inline parsing: fenced code blocks, inline code, bold, italic. No library.
+
+## File Structure
+
+```
+lem-chat/
+├── index.html          # Demo page
+├── src/
+│   ├── lem-chat.ts     # Main container + SSE client
+│   ├── lem-messages.ts # Message list with scroll anchoring
+│   ├── lem-message.ts  # Single message with streaming
+│   ├── lem-input.ts    # Text input
+│   ├── markdown.ts     # Minimal markdown → HTML
+│   └── styles.ts       # CSS template literals
+├── package.json        # typescript + esbuild
+└── tsconfig.json
+```
+
+Build: `esbuild src/lem-chat.ts --bundle --outfile=dist/lem-chat.js`
+
+## Not in v1
+
+- Model selection UI
+- Conversation persistence
+- File/image upload
+- Syntax highlighting
+- Typing indicators
+- User avatars