Browser notifications for research activity across projects and background windows #704

The dashboard currently provides no browser notification when a research chat finishes while another project, tab, or window is being viewed.

Please add browser-local, opt-in notifications for completed agent replies, pending input, and finished/failed runs. The implementation should work on any HTTPS or localhost deployment without hostname-specific configuration, reuse the global event stream, and avoid duplicate alerts from multiple tabs. A running chat should still be tracked when its project has never been opened, and a reconnect should recover completions missed while disconnected.

Related: #359 proposes this feature but currently has merge conflicts. A refreshed implementation based on that work is ready against current main, with cross-tab delivery coordination, chat snapshot recovery, workspace cleanup, current localization catalogs, and tests.

This request concerns notifications while at least one dashboard tab remains open; browser-closed push delivery is outside its scope.