Add extension icons to manifest

Added three standard Chrome extension icon sizes:
- 16x16 for toolbar
- 48x48 for extension management page
- 128x128 for Chrome Web Store and installation dialog

Icons are PNG with transparency, properly sized, and optimized.

Author: pdugan20

chrome-extension/icons/icon128.png

@@ -4,6 +4,12 @@
   "version": "2.0.0",
   "description": "One-click audiobook downloads from Libby.",
 
+  "icons": {
+    "16": "icons/icon16.png",
+    "48": "icons/icon48.png",
+    "128": "icons/icon128.png"
+  },
+
   "browser_specific_settings": {
     "gecko": {
       "id": "libby-downloader@example.com",