feat: dark theme toggle + CPU/memory sparkline trend charts

- ui/theme.py: light/dark QSS stylesheets, persisted to ~/.sshclient/settings.json
- main_window.py: View menu with dark theme checkbox, applies on startup
- ui/widgets.py: SparklineChart widget for mini trend visualization
- MonitorPanel: CPU and memory sparkline charts showing last 60 samples
- Charts cleared on stop/switch host

ui/widgets.py

@@ -356,6 +356,93 @@ class FileBrowser(QWidget):
 # ============================================================
 # 监控面板
 # ============================================================
+class SparklineChart(QWidget):
+    """迷你趋势图：绘制 0-100 的折线图，显示最近 N 个采样点"""
+
+    def __init__(self, title: str = "", color: str = "#4caf50", max_points: int = 60, parent=None):
+        super().__init__(parent)
+        self._title = title
+        self._color = QColor(color)
+        self._max_points = max_points
+        self._data: list = []
+        self.setMinimumHeight(64)
+        self.setMinimumWidth(120)
+
+    def add_value(self, val: float):
+        self._data.append(max(0, min(100, val)))
+        if len(self._data) > self._max_points:
+            self._data = self._data[-self._max_points:]
+        self.update()
+
+    def clear(self):
+        self._data.clear()
+        self.update()
+
+    def paintEvent(self, _ev):
+        from PyQt5.QtGui import QPainter, QPen, QBrush, QPainterPath, QLinearGradient
+        from PyQt5.QtCore import QRectF, QPointF
+
+        p = QPainter(self)
+        p.setRenderHint(QPainter.Antialiasing)
+        w, h = self.width(), self.height()
+        # 背景
+        p.fillRect(0, 0, w, h, QColor("#ffffff08"))
+        # 标题
+        if self._title:
+            p.setPen(QPen(QColor("#888")))
+            p.setFont(QFont("sans-serif", 8))
+            p.drawText(4, 12, self._title)
+        if len(self._data) < 2:
+            p.setPen(QPen(QColor("#666")))
+            p.setFont(QFont("sans-serif", 8))
+            p.drawText(w // 2 - 20, h // 2, "等待数据...")
+            return
+        # 绘图区域（留出标题空间）
+        top = 16
+        bottom = h - 4
+        left = 4
+        right = w - 4
+        plot_w = right - left
+        plot_h = bottom - top
+        # 点坐标
+        n = len(self._data)
+        step = plot_w / max(n - 1, 1)
+        points = []
+        for i, v in enumerate(self._data):
+            x = left + i * step
+            y = bottom - (v / 100.0) * plot_h
+            points.append(QPointF(x, y))
+        # 填充区域
+        fill_path = QPainterPath()
+        fill_path.moveTo(points[0].x(), bottom)
+        for pt in points:
+            fill_path.lineTo(pt)
+        fill_path.lineTo(points[-1].x(), bottom)
+        fill_path.closeSubpath()
+        grad = QLinearGradient(0, top, 0, bottom)
+        c = self._color
+        grad.setColorAt(0, QColor(c.red(), c.green(), c.blue(), 80))
+        grad.setColorAt(1, QColor(c.red(), c.green(), c.blue(), 10))
+        p.fillPath(fill_path, QBrush(grad))
+        # 折线
+        p.setPen(QPen(self._color, 1.5))
+        line_path = QPainterPath()
+        line_path.moveTo(points[0])
+        for pt in points[1:]:
+            line_path.lineTo(pt)
+        p.drawPath(line_path)
+        # 当前值标签
+        cur = self._data[-1]
+        p.setPen(QPen(self._color))
+        p.setFont(QFont("sans-serif", 9, QFont.Bold))
+        label = f"{cur:.1f}%"
+        p.drawText(right - 40, 12, label)
+        # 当前点圆点
+        p.setBrush(QBrush(self._color))
+        p.setPen(QPen(self._color, 0))
+        p.drawEllipse(points[-1], 3, 3)
+
+
 class MonitorPanel(QWidget):
     """实时监控：CPU、内存、磁盘、网络"""
 
@@ -415,6 +502,14 @@ class MonitorPanel(QWidget):
         grid.addWidget(self._build_card("内存", "mem_card"))
         uv.addLayout(grid)
 
+        # CPU / 内存趋势迷你图
+        spark_row = QHBoxLayout()
+        self._spark_cpu = SparklineChart("CPU 趋势", "#4caf50")
+        self._spark_mem = SparklineChart("内存趋势", "#7986cb")
+        spark_row.addWidget(self._spark_cpu)
+        spark_row.addWidget(self._spark_mem)
+        uv.addLayout(spark_row)
+
         # 负载 + 启动时间
         grid2 = QHBoxLayout()
         grid2.addWidget(self._build_card("系统负载", "load_card"))
@@ -685,6 +780,11 @@ class MonitorPanel(QWidget):
                 pass
             self.worker = None
         self._last_net.clear()
+        # 清空趋势图
+        if hasattr(self, "_spark_cpu"):
+            self._spark_cpu.clear()
+        if hasattr(self, "_spark_mem"):
+            self._spark_mem.clear()
 
     def _kick_one_sample(self):
         """启动一个后台 worker 做一次采集；采完用 QTimer 调度下一次"""
@@ -756,6 +856,7 @@ class MonitorPanel(QWidget):
 
         # CPU
         cpu = m.get("cpu", 0)
+        self._spark_cpu.add_value(cpu)
 
         self._value_label("cpu_card").setText(f"{cpu:.1f}%")
         cores = m.get("cores", 1)
@@ -763,6 +864,7 @@ class MonitorPanel(QWidget):
 
         # 内存
         mp = m.get("mem_percent", 0)
+        self._spark_mem.add_value(mp)
         mu = SystemMonitor.format_bytes(m.get("mem_used", 0))
         mt = SystemMonitor.format_bytes(m.get("mem_total", 0))
         self._value_label("mem_card").setText(f"{mp:.1f}%")