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/app/app_test.go

@@ -91,6 +91,14 @@ func (f fakeExports) ExportFileToTarget(src string, target platform.RemovableTar
 	return "", nil
 }
 
+func (f fakeExports) ListScenarioFilesOnRemovableMedia() ([]platform.ScenarioFileOnRemovableMedia, error) {
+	return nil, nil
+}
+
+func (f fakeExports) ReadScenarioFromRemovableMedia(name string) ([]byte, error) {
+	return nil, nil
+}
+
 type fakeRuntime struct {
 	collectFn func(string) (schema.RuntimeHealth, error)
 	dumpFn    func(string) error
@@ -332,6 +340,10 @@ func (f fakeSAT) RunNCCLTests(_ context.Context, baseDir string, gpuIndices []in
 	return "", nil
 }
 
+func (f fakeSAT) RunScenario(_ context.Context, baseDir string, _ platform.ScenarioSpec, _ func(string)) (string, error) {
+	return "", nil
+}
+
 func TestRunNCCLTestsPassesSelectedGPUs(t *testing.T) {
 	t.Parallel()