[ damus-io/notedeck ] fix: toolbar gap fixed for realsies this time ;_;

Paint solid background (black in dark mode, ALMOST_WHITE in light)
behind the toolbar rect to eliminate the intermittent gap that appeared
during show/hide animation. Render Tabs at full TOOLBAR_HEIGHT.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
https://github.com/damus-io/notedeck/commit/2e519cc2d9d85946193af9e0c4f077bfba7ab416