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

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via [Happy](https://happy.engineering)

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

app/models/auth.py

@@ -0,0 +1,40 @@
+"""Authentication models for user management"""
+from pydantic import BaseModel, EmailStr, Field
+from typing import Optional
+from datetime import datetime
+
+
+class UserCreate(BaseModel):
+    """Schema for user registration"""
+    username: str = Field(..., min_length=3, max_length=50)
+    email: Optional[EmailStr] = None
+    password: str = Field(..., min_length=6)
+    full_name: Optional[str] = None
+
+
+class UserLogin(BaseModel):
+    """Schema for user login"""
+    username: str
+    password: str
+
+
+class User(BaseModel):
+    """Schema for user data"""
+    id: str
+    username: str
+    email: Optional[str] = None
+    full_name: Optional[str] = None
+    is_active: bool = True
+    created_at: datetime
+    last_login: Optional[datetime] = None
+
+
+class Token(BaseModel):
+    """Schema for authentication token"""
+    access_token: str
+    token_type: str = "bearer"
+
+
+class UserInDB(User):
+    """Schema for user stored in database (with hashed password)"""
+    hashed_password: str