chore: import upstream snapshot with attribution

docs/agent/coding-standards.md

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+# Coding Standards
+
+> **Frontend and backend coding conventions.**
+
+## Frontend (TypeScript/React)
+
+### Design System
+
+All UI changes MUST follow the **Swiss International Style**. The full design system is published as a portable pack at [`docs/portable/swiss-design-system/`](../portable/swiss-design-system/README.md). The non-negotiable basics:
+
+- Use `font-serif` for headers, `font-mono` for metadata, `font-sans` for body text
+- Color palette: `#F0F0E8` (Canvas), `#000000` (Ink), `#1D4ED8` (Hyper Blue), `#15803D` (Signal Green), `#F97316` (Alert Orange), `#DC2626` (Alert Red), `#4B5563` (Steel Grey)
+- Components: `rounded-none` with 1px black borders and hard shadows
+- See [`tokens.md`](../portable/swiss-design-system/tokens.md), [`components.md`](../portable/swiss-design-system/components.md), and [`anti-patterns.md`](../portable/swiss-design-system/anti-patterns.md) for the full rules
+
+### Naming Conventions
+
+- Use PascalCase for components
+- Use camelCase for helpers
+- Tailwind utility classes for styling
+
+### Textarea Enter Key Fix
+
+All textareas in forms should include `onKeyDown` with `e.stopPropagation()` for Enter key to ensure newlines work correctly:
+
+```tsx
+const handleKeyDown = (e: React.KeyboardEvent<HTMLTextAreaElement>) => {
+  if (e.key === 'Enter') e.stopPropagation();
+};
+```
+
+### Before Committing
+
+1. Run Prettier: `npm run format`
+2. Run linter: `npm run lint`
+
+## Backend (Python/FastAPI)
+
+### General Rules
+
+- Python 3.11+
+- 4-space indents
+- Type hints on ALL functions
+- Async functions for I/O operations (database, LLM calls)
+- Pydantic models for all request/response schemas
+- Prompts go in `app/prompts/templates.py`
+
+### Error Handling
+
+Log detailed errors server-side, return generic messages to clients:
+
+```python
+except Exception as e:
+    logger.error(f"Operation failed: {e}")
+    raise HTTPException(status_code=500, detail="Operation failed. Please try again.")
+```
+
+### Race Conditions
+
+Use `asyncio.Lock()` for shared resource initialization (see `app/pdf.py` for example).
+
+### Mutable Defaults
+
+Always use `copy.deepcopy()` when assigning mutable default values to avoid shared state bugs:
+
+```python
+# Correct
+import copy
+data = copy.deepcopy(DEFAULT_DATA)
+
+# Incorrect - shared state bug
+data = DEFAULT_DATA
+```
+
+### New Service Pattern
+
+Mirror patterns in `app/services/improver.py` for new services.