uSync.Complete: push dialog fix for Safari 26.4

uSync.Complete: the Publisher "Push to server" button now opens its dialog on Safari 26.4, where it was dimming the page and showing nothing (uSync#1110)
Cause was a height: 100% host inside an auto-height <dialog>; WebKit 26.4 resolves that to zero, 26.6 doesn't
Dialog is now sized from a dialog attribute on the host instead of an injected <style> block; sidebar mode unchanged
Reproduced with a static copy of the dialog in Playwright WebKit 26.4 (Playwright 1.59.1) and 26.6
Pushed 17.5.0-prerelease.34 to the nightly feed for a BrowserStack check
Forward merged v17/main into v18/main with the fix, as a merge commit