ui/jobs_view: cap Selected job activity to 3 recent rows, anchor at bottom

Show only the latest 3 activity entries for the selected job instead of the
full in-memory list, and pin the panel to the bottom of the details pane via a
Border so the command output absorbs the remaining vertical space.

Adds fixedHeightLayout (mirroring minWidthLayout) to reserve a stable height
for the activity list. The full per-job history (maxJobLogs) is untouched and
still available in the History view.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/layout.go

@@ -35,3 +35,33 @@ func (l minWidthLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
 		object.Resize(size)
 	}
 }
+
+// fixedHeightLayout forces its contents to a fixed height while leaving the
+// width to the parent container. It is used to reserve a stable amount of space
+// for the activity panel so a neighbouring widget can absorb the rest.
+type fixedHeightLayout struct {
+	height float32
+}
+
+func (l fixedHeightLayout) MinSize(objects []fyne.CanvasObject) fyne.Size {
+	var width float32
+	for _, object := range objects {
+		if !object.Visible() {
+			continue
+		}
+		if min := object.MinSize(); min.Width > width {
+			width = min.Width
+		}
+	}
+	return fyne.NewSize(width, l.height)
+}
+
+func (l fixedHeightLayout) Layout(objects []fyne.CanvasObject, size fyne.Size) {
+	for _, object := range objects {
+		if !object.Visible() {
+			continue
+		}
+		object.Move(fyne.NewPos(0, 0))
+		object.Resize(fyne.NewSize(size.Width, l.height))
+	}
+}