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_packs.go

@@ -17,6 +17,17 @@ func (a *App) RunNvidiaAcceptancePack(baseDir string, logFunc func(string)) (str
 	return a.sat.RunNvidiaAcceptancePack(baseDir, logFunc)
 }
 
+// RunScenario runs a user-authored ScenarioSpec (see
+// platform.ParseScenarioJSON) — the webui Scenario page and `bee run` CLI
+// command both go through this so a scenario run gets the same SAT base
+// directory default as every other pack.
+func (a *App) RunScenario(ctx context.Context, baseDir string, spec platform.ScenarioSpec, logFunc func(string)) (string, error) {
+	if strings.TrimSpace(baseDir) == "" {
+		baseDir = DefaultSATBaseDir
+	}
+	return a.sat.RunScenario(ctx, baseDir, spec, logFunc)
+}
+
 func (a *App) RunNvidiaAcceptancePackResult(baseDir string) (ActionResult, error) {
 	path, err := a.RunNvidiaAcceptancePack(baseDir, nil)
 	body := "Archive written."