feat: Add complete user authentication system with JWT and mandatory login

Implemented a comprehensive authentication system requiring all users to be
logged in to access the web interface. Features include:

Backend:
- JWT-based authentication with 7-day token expiration
- bcrypt password hashing with 72-byte limit handling
- User management with JSON file storage (config/users.json)
- Pydantic models for validation (UserCreate, UserLogin, User, Token)
- Authentication endpoints: register, login, me, logout
- Protected route dependency with HTTPBearer security

Frontend:
- Login/register page with dual-tab interface (/login)
- Client-side authentication check with automatic redirect
- All content hidden by default, shown only after auth validation
- User info display with logout button
- Main content and tabs hidden when not authenticated
- Auto-redirect to /login if token missing or invalid

Security:
- Password truncation to 72 bytes (bcrypt limitation)
- Token verification on each page load
- Automatic logout and redirect on token expiry
- Username-to-SHA256 user ID generation

Dependencies:
- passlib[bcrypt]==1.7.4
- python-jose[cryptography]==3.3.0
- bcrypt<4.0

Generated with [Claude Code](https://claude.com/claude-code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

templates/index.html

@@ -3,6 +3,9 @@
 {% block content %}
 {% include "components/header.html" %}
 
+<!-- Main content - Hidden by default, shown only when authenticated -->
+<div id="main-content" style="display: none;">
+
 {% include "components/home_section.html" %}
 
 <!-- Nouveaux onglets -->
@@ -110,4 +113,8 @@
 </div>
 
 {% include "components/downloads_section.html" %}
+
+</div>
+<!-- End of main-content -->
+
 {% endblock %}