Cache provider favicons instead of re-fetching on every palette keystroke

[ajot]

The command palette rebuilds all <img> tags on every keystroke, each pointing to google.com/s2/favicons?domain=.... The browser caches these so it's not making network requests each time, but it's still creating N new DOM elements per keystroke.

Options:

• Pre-fetch and cache favicon URLs at startup into a Map
• Debounce the palette re-render (also helps general performance)
• Use CSS background images instead of <img> tags so the DOM churn is lower