Every dialog shared one egui area id, "dialog". egui lays an area out in
the size it had the frame before, and the uninstall prompt's button row
was a right-to-left layout placed straight in the column, which fills
whatever height is left. So after Settings or the release notes had been
open, the prompt came up as tall as they were, with its buttons floating
in the middle, and never shrank back.
- Each dialog gets its own area id (`Dialog::key`), so a small prompt
starts from its own size, and so does each feed alert.
- The uninstall prompt's and the feed alert's button rows sit in
`ui.horizontal` like the other dialogs' rows, so they take only a
row's height.
- A UI test opens Settings, closes it, opens the uninstall prompt and
checks the buttons follow the text (they sat 243 px below it before).
Fixes#88
Co-authored-by: YangXZ2008 <141027290+YangXZ2008@users.noreply.github.com>
