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/app/app.go

@@ -75,6 +75,8 @@ type exportManager interface {
 	ExportFileToTarget(src string, target platform.RemovableTarget) (string, error)
 	ListScenarioFilesOnRemovableMedia() ([]platform.ScenarioFileOnRemovableMedia, error)
 	ReadScenarioFromRemovableMedia(name string) ([]byte, error)
+	ListAvailableScenarios() ([]platform.ScenarioFileOnRemovableMedia, error)
+	ReadScenario(name string) ([]byte, error)
 }
 
 type toolManager interface {