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/projects.md

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+# Project Management
+
+## Overview
+
+Projects are the top-level organizational unit in Headquarter. Each project can contain multiple git repositories and serves as a workspace for related development work.
+
+## How to Use
+
+### Creating a Project
+
+1. Navigate to the **Projects** page from the sidebar
+2. Click the **"New Project"** button
+3. Enter a **name** for your project (required)
+4. Optionally add a **description**
+5. Click **"Create Project"**
+
+### Viewing Projects
+
+The Projects page displays all your projects in a card layout showing:
+- Project name
+- Description (if set)
+- Creation date
+- Associated repositories count
+
+### Opening a Project Workspace
+
+Click on any project card to open its **workspace**. The workspace is the default view for a project and shows:
+- Repository file browser
+- Branch selector
+- File viewer
+
+### Editing a Project
+
+1. From the Projects page, click the **menu icon** (⋮) on a project card
+2. Select **"Edit"**
+3. Update the name or description
+4. Click **"Save"**
+
+### Deleting a Project
+
+1. From the Projects page, click the **menu icon** (⋮) on a project card
+2. Select **"Delete"**
+3. Confirm the deletion
+
+**Note:** Deleting a project also deletes all associated repositories and their data. This action cannot be undone.
+
+## Project Workspace
+
+The project workspace is the default view when you click on a project. It provides:
+
+- **Repository Browser**: Navigate files and directories
+- **File Viewer**: View file contents with syntax highlighting
+- **Branch Management**: Switch between branches
+- **Repository Switcher**: Switch between project repositories
+
+See [Repository Workspace](workspace.md) for detailed documentation.
+
+## API Reference
+
+### Endpoints
+
+- `GET /projects` - List all projects
+- `POST /projects` - Create a new project
+- `GET /projects/{id}` - Get project details
+- `PUT /projects/{id}` - Update a project
+- `DELETE /projects/{id}` - Delete a project
+
+See [Projects API](../api/projects.md) for detailed endpoint documentation.
+
+## Related Features
+
+- [Repository Workspace](workspace.md) - Browse and edit repository files
+- [Git Repositories](repositories.md) - Manage project repositories