Initial commit: Cockpit GPU monitor plugin for NVIDIA

Native, self-contained Cockpit page (plain HTML/JS, no build step) that
polls nvidia-smi via cockpit.spawn to show live GPU utilization, memory,
temperature, power, fan speed, ~5min history sparklines, and the list of
processes using the GPU. No background systemd service; data collection
only happens while the page is open.

gpu-monitor/manifest.json

@@ -0,0 +1,15 @@
+{
+    "requires": { "cockpit": "260" },
+
+    "menu": {
+        "index": {
+            "label": "GPU",
+            "order": 15,
+            "keywords": [
+                { "matches": ["nvidia", "gpu", "cuda", "video", "graphics", "vram"] }
+            ]
+        }
+    },
+
+    "content-security-policy": "img-src 'self' data:"
+}