Dropped the OpenSearch "+" item from the engine dropdown and the per-page
OpenSearch scan that fed it (it ran a fetch on every page load). Users add
engines via Settings instead.
- Custom engine dropdown as a floating overlay view (engine-picker.html) that
renders REAL favicons per engine — a native <select> can't show images.
- Larger catalog (DuckDuckGo/Google/Brave/Bing/Startpage/Yandex/Ecosia/Mojeek/
SearXNG/Wikipedia/Perplexity); Settings has a favicon checklist to choose
which appear in the dropdown (enabledEngines), plus manual add/remove.
- OpenSearch "scan": pages advertising a search engine surface an
"Add <site>'s search" entry in the dropdown.
- Toolbar search box now just a magnifier button that opens the dropdown
(no per-engine icon in the bar).
