Add Chrome extension icons

- Created icon files in 4 sizes: 16x16, 32x32, 48x48, 128x128
- Added icons directory with gradient purple design
- Updated manifest.json to include default_icon configuration
- Icons will now display properly in Chrome toolbar

Required for Chrome Web Store compliance

manifest.json

@@ -28,7 +28,13 @@
   },
   "action": {
     "default_popup": "popup.html",
-    "default_title": "Manga Mass Downloader"
+    "default_title": "Manga Mass Downloader",
+    "default_icon": {
+      "16": "icons/icon16.png",
+      "32": "icons/icon32.png",
+      "48": "icons/icon48.png",
+      "128": "icons/icon128.png"
+    }
   },
   "web_accessible_resources": [
     {