Full-page capture rebuilt - it now works on pages where it used to fail or produce a broken image.

Fixed:
- Pages whose content scrolls inside a panel rather than the window itself (web apps, dashboards, docs sites) captured only the first screen. They now capture in full.
- Sites using smooth scrolling produced torn or duplicated screenshots. Slices are now taken at the exact right position every time.
- Captures were cut short on pages that load content as you scroll. The capture now follows the page as it grows.
- Very long pages failed outright. They now save at a slightly reduced scale instead.
- A capture that failed part-way through did nothing at all, with no explanation. The reason is now shown on the page.
- Pages that stop scrolling partway no longer leave a band of blank space at the bottom of the image.

Also in this release:
- Long captures use much less memory.
- The add-on is about 20% smaller - an unused PDF library dependency has been removed.
- Now requires Firefox 142 or later.