Add dark mode support and replace group assignment with playlist assignment

- Added comprehensive dark mode styling to all pages:
  * Dashboard (workflow guide, secondary text, log items)
  * Admin panel with user management system
  * Content/playlist management page
  * Upload media page
  * Add player page

- Implemented user management system:
  * Create/edit/delete users
  * Two role types (user/admin)
  * Reset password functionality
  * Role-based permissions

- Replaced group assignment with playlist assignment:
  * Players now directly assigned to playlists
  * Updated add player form and backend
  * Removed group selection from player creation

- Fixed bugs:
  * Updated instance_path configuration for SQLite
  * Fixed import path in app factory
  * Updated dependencies (Pillow 11.0.0, removed gevent)

- Added start.sh script for easy development server launch

app/templates/content/upload_media.html

@@ -19,17 +19,32 @@
         cursor: pointer;
     }
     
+    body.dark-mode .upload-zone {
+        background: #1a202c;
+        border-color: #4a5568;
+    }
+    
     .upload-zone:hover {
         border-color: #667eea;
         background: #f0f2ff;
     }
     
+    body.dark-mode .upload-zone:hover {
+        border-color: #7c3aed;
+        background: #2d3748;
+    }
+    
     .upload-zone.dragover {
         border-color: #667eea;
         background: #e8ebff;
         transform: scale(1.02);
     }
     
+    body.dark-mode .upload-zone.dragover {
+        border-color: #7c3aed;
+        background: #2d3748;
+    }
+    
     .file-input-wrapper {
         margin: 20px 0;
     }
@@ -53,6 +68,11 @@
         margin-bottom: 10px;
     }
     
+    body.dark-mode .file-item {
+        background: #2d3748;
+        border-color: #4a5568;
+    }
+    
     .file-info {
         display: flex;
         align-items: center;
@@ -87,6 +107,10 @@
         color: #333;
     }
     
+    body.dark-mode .form-group label {
+        color: #e2e8f0;
+    }
+    
     .form-control {
         width: 100%;
         padding: 12px;
@@ -95,12 +119,23 @@
         font-size: 14px;
     }
     
+    body.dark-mode .form-control {
+        background: #1a202c;
+        border-color: #4a5568;
+        color: #e2e8f0;
+    }
+    
     .form-control:focus {
         outline: none;
         border-color: #667eea;
         box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.1);
     }
     
+    body.dark-mode .form-control:focus {
+        border-color: #7c3aed;
+        box-shadow: 0 0 0 3px rgba(124, 58, 237, 0.2);
+    }
+    
     .btn-upload {
         background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
         color: white;
@@ -125,6 +160,26 @@
         transform: none;
     }
     
+    /* Dark mode text colors */
+    body.dark-mode h1,
+    body.dark-mode h2,
+    body.dark-mode h3 {
+        color: #e2e8f0;
+    }
+    
+    body.dark-mode p,
+    body.dark-mode small {
+        color: #a0aec0;
+    }
+    
+    body.dark-mode .file-info > div > div {
+        color: #e2e8f0;
+    }
+    
+    body.dark-mode .file-info > div > div:last-child {
+        color: #718096;
+    }
+    
     .playlist-selector {
         background: #f8f9fa;
         padding: 20px;
@@ -133,10 +188,20 @@
         border: 2px solid #dee2e6;
     }
     
+    body.dark-mode .playlist-selector {
+        background: #1a202c;
+        border-color: #4a5568;
+    }
+    
     .playlist-selector.selected {
         border-color: #667eea;
         background: #f0f2ff;
     }
+    
+    body.dark-mode .playlist-selector.selected {
+        border-color: #7c3aed;
+        background: #2d3748;
+    }
 </style>
 
 <div class="upload-container">