perf(browser): move favicon probe off the page-load critical path

#414 in DioxusLabs/blitz — merged 2026-05-03

Page navigations were blocking on a favicon fetch plus a full image decode before swapping in the new document, adding a round-trip of latency to every load. This moves the probe into a background task spawned after the document is applied, with stale-probe guards on the tab and id-keyed patching of history entries so a late-arriving icon still updates the right row.