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_install.go

@@ -28,6 +28,20 @@ func (a *App) ReadScenarioFromRemovableMedia(name string) ([]byte, error) {
 	return a.exports.ReadScenarioFromRemovableMedia(name)
 }
 
+// ListAvailableScenarios lists every scenario runnable via ReadScenario:
+// shipped with the image plus anything found on removable media — see
+// platform.System.ListAvailableScenarios.
+func (a *App) ListAvailableScenarios() ([]platform.ScenarioFileOnRemovableMedia, error) {
+	return a.exports.ListAvailableScenarios()
+}
+
+// ReadScenario resolves scenarios/<name>.json, preferring the copy shipped
+// with the image over one on removable media — see
+// platform.System.ReadScenario.
+func (a *App) ReadScenario(name string) ([]byte, error) {
+	return a.exports.ReadScenario(name)
+}
+
 func (a *App) ExportLatestAudit(target platform.RemovableTarget) (string, error) {
 	if _, err := os.Stat(DefaultAuditJSONPath); err != nil {
 		return "", err