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/webui/layout.go

@@ -108,6 +108,7 @@ func layoutNav(active string, buildLabel string) string {
 		{id: "load", label: "3. Load", href: "/load"},
 		{id: "burn", label: "4. Burn", href: "/burn"},
 		{id: "benchmark", label: "5. Benchmark", href: "/benchmark"},
+		{id: "scenario", label: "6. Scenario", href: "/scenario"},
 		{sep: true},
 		{id: "tasks", label: "Tasks", href: "/tasks"},
 		{id: "tools", label: "Tools", href: "/tools"},