Add server-side metrics ring buffer, seed dashboard charts from server history

Background task samples system (CPU/RAM/GPU) and per-target (FPS/timing) metrics
every 1s into a 120-sample ring buffer (~2 min). New API endpoint
GET /system/metrics-history returns the buffer. Dashboard charts now seed from
server history on load instead of sessionStorage, surviving page refreshes.

Also removes emoji from brightness source labels.

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

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

@@ -6,11 +6,12 @@ from datetime import datetime
 from typing import Optional
 
 import psutil
-from fastapi import APIRouter, HTTPException, Query
+from fastapi import APIRouter, Depends, HTTPException, Query
 from pydantic import BaseModel
 
 from wled_controller import __version__
 from wled_controller.api.auth import AuthRequired
+from wled_controller.api.dependencies import get_processor_manager
 from wled_controller.api.schemas.system import (
     DisplayInfo,
     DisplayListResponse,
@@ -192,6 +193,19 @@ def get_system_performance(_: AuthRequired):
     )
 
 
+@router.get("/api/v1/system/metrics-history", tags=["Config"])
+async def get_metrics_history(
+    _: AuthRequired,
+    manager=Depends(get_processor_manager),
+):
+    """Return the last ~2 minutes of system and per-target metrics.
+
+    Used by the dashboard to seed charts on page load so history
+    survives browser refreshes.
+    """
+    return manager.metrics_history.get_history()
+
+
 # ---------------------------------------------------------------------------
 # ADB helpers (for Android / scrcpy engine)
 # ---------------------------------------------------------------------------