Add working frontend prototype with edit-in-place functionality

✨ Features implemented:
- Complete demo website (Acme Consulting Services)
- Core insertr.js library with edit-in-place functionality
- Support for simple text and rich markdown content
- Mock authentication system (login/logout simulation)
- Edit mode toggle for client interface
- Local storage persistence
- Visual feedback for saving content
- Multi-page support (index.html, about.html)
- Responsive design with clean UI

🎯 Three user experiences:
- Customer: Clean website with no edit interface
- Client: Same website + edit buttons when authenticated
- Developer: Simple class-based integration

🏗️ Architecture:
- Vanilla JavaScript (no framework dependencies)
- Alpine.js ready for enhanced interactivity
- CSS-only styling for edit indicators
- Mock API structure for backend planning

Next: Connect to Go backend with real persistence

demo-site/README.md

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+# Insertr Demo Site
+
+This is a prototype demonstration of the Insertr edit-in-place CMS system.
+
+## What is Insertr?
+
+Insertr allows developers to make any website content editable by simply adding a CSS class. Clients can then log in and edit content directly on their website without needing to learn a complex admin interface.
+
+## Three User Types
+
+### 1. The Customer (End User)
+- Sees a clean, professional website
+- No editing interface visible
+- Fast loading with minimal overhead
+
+### 2. The Client (Content Manager)  
+- Logs in to see the same website with subtle edit buttons
+- Clicks edit buttons to modify content inline
+- Can edit both simple text and rich markdown content
+- Changes are saved immediately
+
+### 3. The Developer (You)
+- Simple integration: just add `class="insertr"` and `data-content-id="unique-id"`
+- No complex setup or framework dependencies
+- Works with any existing website
+
+## Demo Instructions
+
+1. **Open `index.html` in your browser** - You'll see the customer view
+2. **Click "Login as Client"** - This simulates authentication 
+3. **Click "Edit Mode: Off" to turn on editing** - Now you'll see edit buttons (✏️) appear
+4. **Click any edit button** to modify content
+5. **Try both simple text and rich content** (marked with 📝)
+6. **Navigate to `about.html`** to see how it works across pages
+
+## Technical Details
+
+### For Developers
+
+To make content editable, just add the insertr class and data attribute:
+
+```html
+<!-- Simple text content -->
+<div class="insertr" data-content-id="hero-title">
+  <h1>Your Website Title</h1>
+</div>
+
+<!-- Rich content with markdown -->
+<div class="insertr" data-content-id="about-text" data-content-type="rich">
+  <h2>About Us</h2>
+  <p>We help businesses succeed...</p>
+</div>
+```
+
+### Integration
+
+Include the Insertr library in your HTML:
+
+```html
+<link rel="stylesheet" href="insertr/insertr.css">
+<script src="insertr/insertr.js"></script>
+```
+
+That's it! The library will automatically scan for editable elements and set up the editing interface.
+
+## Current Features
+
+- ✅ Edit-in-place for simple text content  
+- ✅ Markdown editing for rich content
+- ✅ Mock authentication (login/logout)
+- ✅ Edit mode toggle
+- ✅ Local storage persistence
+- ✅ Visual feedback for saving
+- ✅ Multi-page support
+- ✅ Responsive design
+
+## Planned Features
+
+- [ ] Real backend API integration
+- [ ] Authentik OAuth integration  
+- [ ] File upload and image management
+- [ ] Content versioning and rollback
+- [ ] Multi-user permissions
+- [ ] Admin dashboard
+- [ ] Git-based deployment
+
+## Architecture
+
+This prototype demonstrates the frontend experience. The full system will include:
+
+- **Go backend** with REST API
+- **File-based content storage** with Git versioning
+- **Authentik OAuth** for secure authentication
+- **Multi-tenant support** for hosting multiple client sites
+- **Developer tools** for easy integration
+
+## Files Structure
+
+```
+demo-site/
+├── index.html              # Homepage demo
+├── about.html              # Additional page demo  
+├── assets/
+│   └── style.css          # Demo site styling
+├── insertr/
+│   ├── insertr.js         # Core library
+│   ├── insertr.css        # Edit interface styling
+│   └── components/        # Future: edit components
+└── mock-api/
+    └── content.json       # Mock backend data structure
+```
+
+## Try It Now!
+
+Open `index.html` in your browser and experience the three different user views by using the authentication controls in the top right corner.