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>

static/style.css

@@ -70,6 +70,17 @@ body {
     background-color: var(--border-color);
 }
 
+.nav-user {
+    display: flex;
+    align-items: center;
+    gap: 1rem;
+}
+
+.nav-user span {
+    color: var(--text-primary);
+    font-weight: 500;
+}
+
 /* Container */
 .container {
     max-width: 1400px;
@@ -528,6 +539,80 @@ body {
     color: var(--primary-color);
 }
 
+/* Auth Pages */
+.auth-page {
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    min-height: calc(100vh - 200px);
+    padding: 2rem;
+}
+
+.auth-container {
+    max-width: 450px;
+    width: 100%;
+    background-color: var(--card-background);
+    padding: 3rem;
+    border-radius: 8px;
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
+}
+
+.auth-container h2 {
+    margin-bottom: 0.5rem;
+    text-align: center;
+}
+
+.auth-subtitle {
+    text-align: center;
+    color: var(--text-secondary);
+    margin-bottom: 2rem;
+}
+
+.auth-form {
+    margin-bottom: 1.5rem;
+}
+
+.checkbox-group {
+    display: flex;
+    align-items: center;
+}
+
+.checkbox-label {
+    display: flex;
+    align-items: center;
+    cursor: pointer;
+    color: var(--text-secondary);
+}
+
+.checkbox-label input[type="checkbox"] {
+    margin-right: 0.5rem;
+    cursor: pointer;
+}
+
+.btn-block {
+    width: 100%;
+}
+
+.auth-footer {
+    text-align: center;
+    padding-top: 1.5rem;
+    border-top: 1px solid var(--border-color);
+}
+
+.auth-footer p {
+    color: var(--text-secondary);
+    margin: 0;
+}
+
+.auth-footer a {
+    color: var(--primary-color);
+    text-decoration: none;
+}
+
+.auth-footer a:hover {
+    text-decoration: underline;
+}
+
 /* Footer */
 .footer {
     background-color: var(--secondary-color);