Fixed a bug where the time-conversion bubble could render off-screen (clipped or invisible) when hovering a timestamp near the top of the page. The bubble now automatically flips to appear below the text when there isn't enough room above it, and is also clamped so it can't run off the left or right edge of the window.

Clicking the toolbar icon now opens the settings page directly.
Added a 128×128 icon.
Performance fix: prevented repeated re-scanning of already-converted timestamps on pages that update frequently.