Add real-time system performance charts to dashboard

Backend: GET /api/v1/system/performance endpoint using psutil (CPU/RAM)
and nvidia-ml-py (GPU utilization, memory, temperature) with graceful
fallback. Frontend: Chart.js line charts with rolling 60-sample history
persisted to sessionStorage, flicker-free updates via persistent DOM
and diff-based dynamic section refresh.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/schemas/system.py

@@ -47,3 +47,24 @@ class ProcessListResponse(BaseModel):
 
     processes: List[str] = Field(description="Sorted list of unique process names")
     count: int = Field(description="Number of unique processes")
+
+
+class GpuInfo(BaseModel):
+    """GPU performance information."""
+
+    name: str | None = Field(default=None, description="GPU device name")
+    utilization: float | None = Field(default=None, description="GPU core usage percent")
+    memory_used_mb: float | None = Field(default=None, description="GPU memory used in MB")
+    memory_total_mb: float | None = Field(default=None, description="GPU total memory in MB")
+    temperature_c: float | None = Field(default=None, description="GPU temperature in Celsius")
+
+
+class PerformanceResponse(BaseModel):
+    """System performance metrics."""
+
+    cpu_percent: float = Field(description="System-wide CPU usage percent")
+    ram_used_mb: float = Field(description="RAM used in MB")
+    ram_total_mb: float = Field(description="RAM total in MB")
+    ram_percent: float = Field(description="RAM usage percent")
+    gpu: GpuInfo | None = Field(default=None, description="GPU info (null if unavailable)")
+    timestamp: datetime = Field(description="Measurement timestamp")