feat: Add opt-in AI Chat Assistant dock panel (OpenAI-compatible API)

[Vickyrrrrrr]

What this PR does

Adds an AI Chat Assistant as an optional, dockable panel inside CQ-editor.
Users can describe what they want to model in plain English, receive valid
CadQuery code from any OpenAI-compatible LLM, and insert it directly into
the editor — all without leaving the app.

Motivation

CadQuery is powerful but has a steep learning curve for new users. An
in-editor AI assistant lowers that barrier significantly: instead of
looking up API docs, users can describe intent ("make a box with filleted
edges") and get runnable code instantly. Power users benefit too — they
can ask the AI to iterate on an existing model using the current script
as context.

Changes

cq_editor/widgets/ai_chat.py (new file)

• AIChatWidget — dockable QWidget, integrates like any existing panel
• Chat history with colour-coded roles (You / AI / Error)
• Context injection: current editor script is prepended to every prompt
  so the LLM edits the existing model rather than starting from scratch
• Async LLM calls via QThread — UI never freezes
• Insert & Run button: extracts code from reply, pushes to editor,
  optionally triggers an immediate re-render
• preferences Parameter tree: Enabled, Provider URL, Model, API Key,
  Auto-run — appears under Edit → Preferences → AI Assistant

cq_editor/main_window.py (minimal, additive changes only)

• Register AIChatWidget as a dock panel (right side, default)
• Wire insert_code signal → editor.set_text
• Add AI Assistant toggle under Tools menu

How to use

1. pip install openai
2. Edit → Preferences → AI Assistant → enter API Key + Model
3. Tools → AI Assistant
4. Type a prompt, click Insert & Run

Works with OpenAI, Anthropic (via OpenRouter), local Ollama, or any
OpenAI-compatible endpoint via the Provider URL field.

Compatibility

• openai is optional — guarded by try/except ImportError; CQ-editor
  starts normally without it
• Zero changes to existing behaviour or existing widgets
• All new logic is self-contained in ai_chat.py

Testing

• CQ-editor launches normally with and without openai installed
• Panel docks, undocks, and toggles via View menu
• Preferences save and restore across sessions
• Context injection sends current script to LLM
• Insert & Run loads code into editor and triggers re-render
• Tested with OpenAI (gpt-4o, o3) and OpenRouter

Future work (follow-up PRs)

• Streaming token-by-token response display
• Diff-based code patching instead of full replace
• Auto-fix: feed traceback back to LLM on render error