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>

iso/overlay/usr/share/bee/scenarios/nvbandwidth-all-gpu-power-watch.json

@@ -0,0 +1,24 @@
+{
+  "name": "nvbandwidth-all-gpu-power-watch",
+  "timeout_sec": 1800,
+  "jobs": [
+    {
+      "name": "ipmi-sensors",
+      "type": "sampler",
+      "interval_sec": 2,
+      "cmd": ["ipmitool", "sensor"]
+    },
+    {
+      "name": "gpu-power",
+      "type": "sampler",
+      "interval_sec": 2,
+      "cmd": ["nvidia-smi", "--query-gpu=index,power.draw,temperature.gpu,clocks.sm", "--format=csv"]
+    },
+    {
+      "name": "nvbandwidth-all",
+      "type": "command",
+      "cmd": ["dcgmi", "diag", "-r", "nvbandwidth", "-i", "{{gpus}}"],
+      "gpu_indices": [0, 1, 2, 3, 4, 5]
+    }
+  ]
+}