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.go

@@ -73,6 +73,8 @@ type serviceManager interface {
 type exportManager interface {
 	ListRemovableTargets() ([]platform.RemovableTarget, error)
 	ExportFileToTarget(src string, target platform.RemovableTarget) (string, error)
+	ListScenarioFilesOnRemovableMedia() ([]platform.ScenarioFileOnRemovableMedia, error)
+	ReadScenarioFromRemovableMedia(name string) ([]byte, error)
 }
 
 type toolManager interface {
@@ -154,6 +156,7 @@ type satRunner interface {
 	RunFanStressTest(ctx context.Context, baseDir string, opts platform.FanStressOptions) (string, error)
 	RunPlatformStress(ctx context.Context, baseDir string, opts platform.PlatformStressOptions, logFunc func(string)) (string, error)
 	RunNCCLTests(ctx context.Context, baseDir string, gpuIndices []int, logFunc func(string)) (string, error)
+	RunScenario(ctx context.Context, baseDir string, spec platform.ScenarioSpec, logFunc func(string)) (string, error)
 }
 
 type runtimeChecker interface {