platform/app/webui: ship the power-watch scenario baked into the image

platform.LocalScenariosDir (/usr/share/bee/scenarios, populated from
iso/overlay/usr/share/bee/scenarios/ by build.sh's overlay rsync) is now
checked before removable media for both `bee run <name>` and the "6.
Scenario" page — a scenario shipped with the image works with no USB
stick required. ReadScenario/ListAvailableScenarios merge local + USB;
the removable-media-only functions from the previous commit are kept
as-is (still used directly where that's actually what's wanted) rather
than renamed out from under existing callers/tests.

iso/overlay/usr/share/bee/scenarios/nvbandwidth-all-gpu-power-watch.json
is a copy of scenarios/nvbandwidth-all-gpu-power-watch.json — the two
aren't auto-synced (documented in scenarios/README.md), so shipping a
scenario baked-in means checking it into both places.

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

audit/internal/webui/api.go

@@ -582,7 +582,7 @@ func (h *handler) handleAPIScenarioList(w http.ResponseWriter, _ *http.Request)
 		writeError(w, http.StatusServiceUnavailable, "app not configured")
 		return
 	}
-	files, err := h.opts.App.ListScenarioFilesOnRemovableMedia()
+	files, err := h.opts.App.ListAvailableScenarios()
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return