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_test.go

@@ -99,6 +99,14 @@ func (f fakeExports) ReadScenarioFromRemovableMedia(name string) ([]byte, error)
 	return nil, nil
 }
 
+func (f fakeExports) ListAvailableScenarios() ([]platform.ScenarioFileOnRemovableMedia, error) {
+	return nil, nil
+}
+
+func (f fakeExports) ReadScenario(name string) ([]byte, error) {
+	return nil, nil
+}
+
 type fakeRuntime struct {
 	collectFn func(string) (schema.RuntimeHealth, error)
 	dumpFn    func(string) error