ui: a badge bar with a Trigger badge, and the trigger form in the slide-over

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui_widgets.cpp

@@ -74,6 +74,11 @@ void install_css() {
       ".udpscope-slideover { background-color: @window_bg_color; border-left: 1px solid @borders;"
       " box-shadow: -3px 0 10px rgba(0, 0, 0, 0.35); }\n"
       ".udpscope-slideover-title { font-weight: bold; }\n"
+      // The badge bar: a badge is a pill carrying a name and a state; the one whose page is open is
+      // marked, as a bench scope marks the open menu's button.
+      ".udpscope-badge { padding: 2px 12px; min-height: 0; border-radius: 14px; }\n"
+      ".udpscope-badge-open { background-color: alpha(@accent_bg_color, 0.28); }\n"
+      ".udpscope-badge-state { opacity: 0.7; }\n"
       // The contextual-help "(i)": a *mark*, not a button. Adwaita's menu button is a 34x34 control
       // (a 24 px floor plus 5 px of padding) with a 16 px glyph, which beside a heading reads as a
       // second toolbar button. Measured on libadwaita 1.10: the outer `menubutton` node *and* its