fix: use the bundled search engine icon

The search provider's favicon_url pointed at zavage-software.com, so Firefox
downloaded the icon from our server whenever it installed or updated the
engine. That contradicted the privacy policy's "Nothing is reported to Zavage
Software", would break if the site moved, and blocks a future MV3 move, which
requires a packaged favicon.

Without favicon_url, Firefox uses the manifest icons instead
(AddonSearchEngine#initFromManifest). The remote file was byte-identical to
icons/powered-by-openai-icon-48x48.png, so the artwork is unchanged.

Closes #20

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

manifest.json

@@ -59,7 +59,6 @@
       "name": "ChatGPT",
       "search_url": "https://chatgpt.com?q={searchTerms}",
       "keyword": "gpt",
-      "favicon_url": "https://zavage-software.com/static/ChatGPT-favicon.png",
       "is_default": false,
       "encoding": "UTF-8"
     }