webui: add "6. Scenario" page — run scriptable test scenarios from removable media through the normal task queue

Exposes the scenario engine (platform.System.RunScenario, added earlier)
in the web UI instead of only the `bee run` CLI: a new nav item lists every
scenarios/*.json found on mounted removable media (GET /api/scenario/list)
and runs one with a click (POST /api/scenario/run), enqueued as a normal
Task with target "scenario" — progress/logs live in Tasks like any other
SAT pack, no separate live-output UI needed.

- app.go: satRunner gains RunScenario, exportManager gains
  ListScenarioFilesOnRemovableMedia/ReadScenarioFromRemovableMedia — both
  already implemented on platform.System, just newly exposed through App.
- webui/tasks.go: taskParams.ScenarioName; runTask's "scenario" case reads
  the file from removable media, parses it, and runs it.
- webui/page_scenario.go: the page itself.

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

audit/internal/platform/export.go

@@ -203,3 +203,47 @@ func (s *System) ReadScenarioFromRemovableMedia(name string) ([]byte, error) {
 	}
 	return nil, fmt.Errorf("scenarios/%s.json not found on any removable media", name)
 }
+
+// ScenarioFileOnRemovableMedia is one scenarios/*.json file found on a
+// mounted removable target, as returned by ListScenarioFilesOnRemovableMedia.
+type ScenarioFileOnRemovableMedia struct {
+	Name   string // filename without ".json" — what ReadScenarioFromRemovableMedia/bee run expects
+	Device string // which removable target it was found on
+}
+
+// ListScenarioFilesOnRemovableMedia mounts each removable target in turn
+// (unmounting again afterward if it mounted it itself) and collects every
+// *.json file under a top-level scenarios/ directory. Used by the webui's
+// Scenario page to show what's available to run without requiring the
+// operator to already know a file's exact name.
+func (s *System) ListScenarioFilesOnRemovableMedia() ([]ScenarioFileOnRemovableMedia, error) {
+	targets, err := s.ListRemovableTargets()
+	if err != nil {
+		return nil, err
+	}
+	var out []ScenarioFileOnRemovableMedia
+	for _, target := range targets {
+		mountpoint, mountedHere, mountErr := mountRemovableTargetReadOnly(target)
+		if mountErr != nil {
+			continue
+		}
+		entries, readErr := os.ReadDir(filepath.Join(mountpoint, "scenarios"))
+		if readErr == nil {
+			for _, e := range entries {
+				if e.IsDir() || !strings.HasSuffix(e.Name(), ".json") {
+					continue
+				}
+				out = append(out, ScenarioFileOnRemovableMedia{
+					Name:   strings.TrimSuffix(e.Name(), ".json"),
+					Device: target.Device,
+				})
+			}
+		}
+		if mountedHere {
+			_, _ = exportExecCommand("umount", mountpoint).CombinedOutput()
+			_ = os.Remove(mountpoint)
+		}
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Name < out[j].Name })
+	return out, nil
+}