Implement complete user authentication system

- Configured Flask-Login with user_loader
- Added register, login, logout routes with proper validation
- Created login.html and register.html templates with auth forms
- Updated base.html navigation to show username and conditional menu
- Added auth page styling to style.css
- Protected all routes with @login_required decorator
- Updated all routes to filter by current_user.id
- Added user ownership validation for:
  - Channels (can only view/refresh own channels)
  - Videos (can only watch/download own videos)
  - Streams (can only stream videos from own channels)
- Updated save_to_db() calls to pass current_user.id
- Improved user_loader to properly handle session management

Features:
- User registration with password confirmation
- Secure password hashing with bcrypt
- Login with "remember me" functionality
- Flash messages for all auth actions
- Redirect to requested page after login
- User-specific data isolation (multi-tenant)

Security:
- All sensitive routes require authentication
- Users can only access their own data
- Passwords hashed with bcrypt salt
- Session-based authentication via Flask-Login

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

templates/login.html

@@ -0,0 +1,54 @@
+{% extends "base.html" %}
+
+{% block title %}Login - YottoB{% endblock %}
+
+{% block content %}
+<div class="auth-page">
+    <div class="auth-container">
+        <h2>Login to YottoB</h2>
+        <p class="auth-subtitle">Access your YouTube video collection</p>
+
+        <form method="POST" action="{{ url_for('login') }}" class="auth-form">
+            <div class="form-group">
+                <label for="username">Username</label>
+                <input
+                    type="text"
+                    id="username"
+                    name="username"
+                    required
+                    autofocus
+                    class="form-input"
+                    placeholder="Enter your username"
+                >
+            </div>
+
+            <div class="form-group">
+                <label for="password">Password</label>
+                <input
+                    type="password"
+                    id="password"
+                    name="password"
+                    required
+                    class="form-input"
+                    placeholder="Enter your password"
+                >
+            </div>
+
+            <div class="form-group checkbox-group">
+                <label class="checkbox-label">
+                    <input type="checkbox" name="remember" id="remember">
+                    <span>Remember me</span>
+                </label>
+            </div>
+
+            <div class="form-actions">
+                <button type="submit" class="btn btn-primary btn-block">Login</button>
+            </div>
+        </form>
+
+        <div class="auth-footer">
+            <p>Don't have an account? <a href="{{ url_for('register') }}">Register here</a></p>
+        </div>
+    </div>
+</div>
+{% endblock %}