platform/webui: add scenario description, show it in the Scenario page's list

ScenarioSpec gains an optional "description" field. Listing (both
ListLocalScenarioFiles and ListScenarioFilesOnRemovableMedia, via the new
scenarioDescription helper) reads it out of each file without requiring
full ParseScenarioJSON validation to succeed, so a listing never hides a
scenario over an unrelated validation issue. The webui Scenario page now
renders Name/Description/Found-on/Run instead of just Name/Found-on — a
bare filename rarely tells anyone but the author what a scenario actually
does.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

audit/internal/platform/export_test.go

@@ -234,7 +234,8 @@ func TestReadScenarioFallsBackToRemovableMediaWhenNotLocal(t *testing.T) {
 
 func TestListAvailableScenariosMergesLocalAndRemovable(t *testing.T) {
 	localDir := withLocalScenariosDir(t)
-	if err := os.WriteFile(filepath.Join(localDir, "shipped.json"), []byte(`{}`), 0644); err != nil {
+	shipped := []byte(`{"name":"shipped","description":"reproduces the reboot"}`)
+	if err := os.WriteFile(filepath.Join(localDir, "shipped.json"), shipped, 0644); err != nil {
 		t.Fatalf("write local scenario: %v", err)
 	}
 
@@ -265,7 +266,13 @@ func TestListAvailableScenariosMergesLocalAndRemovable(t *testing.T) {
 	if got[0].Name != "shipped" || got[0].Device != "local (shipped with image)" {
 		t.Fatalf("got[0]=%+v want local shipped entry first", got[0])
 	}
+	if got[0].Description != "reproduces the reboot" {
+		t.Fatalf("got[0].Description=%q want %q", got[0].Description, "reproduces the reboot")
+	}
 	if got[1].Name != "from-usb" {
 		t.Fatalf("got[1]=%+v want from-usb", got[1])
 	}
+	if got[1].Description != "" {
+		t.Fatalf("got[1].Description=%q want empty (file has no description field)", got[1].Description)
+	}
 }