Stop section headers rendering clipped at the top of a list

JetBrainsMono Nerd Font draws 10% of the em above the ascent Text
reserves for it (yMax 1120 against ascent 1020), and a patched or
user-chosen family can be worse. In normal flow that sliver costs
nothing; at the top of a clipping list it is sliced off and the header
renders beheaded.

Reserving it in PanelSectionHeader covers every panel at once, rather
than each list working around it in its own geometry.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

shell/Ui/PanelSectionHeader.qml

@@ -15,4 +15,12 @@ Text {
   font.family: fontFamily
   font.pixelSize: fontSize
   font.bold: true
+
+  // Glyphs can paint above the box Text reserves for them: JetBrainsMono Nerd
+  // Font's outlines run 10% of the em past its own ascent, and a patched or
+  // user-chosen family can be worse. That sliver is invisible in normal flow,
+  // but a header sitting at the top of a clipping list — bluetooth's device
+  // list, network's station list — loses it to the clip and renders beheaded.
+  // Reserve the overshoot here so every panel is covered at once.
+  topPadding: Math.ceil(fontSize * 0.15)
 }