docs: comprehensive documentation overhaul

Add complete documentation structure:
- Frontend architecture documentation
- Database schema documentation
- Deployment guides (Docker, Traefik, Authentik, Environment)
- Development guides (Setup, Testing, Contributing, Quality Gates)
- Deployment architecture documentation
- Updated docs README with complete navigation

All new features and APIs are now documented.
Quality gates: docs only, no code changes

docs/features/settings.md

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+# User Settings
+
+## Overview
+
+User settings allow you to customize your Headquarter experience, including theme preferences and git identity.
+
+## How to Use
+
+### Accessing Settings
+
+1. Click your **name** in the header
+2. Select **"Settings"** from the dropdown
+
+### Theme Selection
+
+Choose your preferred theme:
+
+- **System** - Follows your operating system preference
+- **Light** - Light color scheme
+- **Dark** - Dark color scheme
+
+Changes are applied immediately and persist across sessions.
+
+### Git Identity
+
+Configure your git identity for commits made through the workspace:
+
+- **Name**: Your display name for git commits
+- **Email**: Your email for git commits
+
+This information is used when you make quick edits in the repository workspace.
+
+### Default Editor
+
+Choose your preferred editor for quick edits:
+
+- Options depend on available tool types
+- Used when opening files for editing
+
+## Configuration
+
+Settings are stored per-user in the database and persist across sessions.
+
+## API Reference
+
+### Endpoints
+
+- `GET /users/me/config` - Get user settings
+- `PATCH /users/me/config` - Update user settings
+
+See [Users API](../api/users.md) for detailed endpoint documentation.
+
+## Related Features
+
+- [Authentication](auth.md) - User authentication
+- [Repository Workspace](workspace.md) - Edit files with git identity