ui: a slide-over panel over the plot's right edge, with its probe

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

src/ui_widgets.cpp

@@ -69,6 +69,11 @@ void install_css() {
       ".udpscope-tour-spot { border: 2px solid @accent_bg_color; border-radius: 8px; }\n"
       ".udpscope-tour-card { background-color: @window_bg_color; border: 1px solid @borders;"
       " border-radius: 12px; padding: 12px; box-shadow: 0 3px 12px rgba(0, 0, 0, 0.45); }\n"
+      // The slide-over panel over the plot's right edge: opaque (it must hide the plot under it),
+      // with an edge and a shadow so it reads as above the plot rather than beside it.
+      ".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 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