Use flat buttons and power icon for dashboard start/stop actions

- Replace btn-icon with transparent flat dashboard-action-btn style
- Use Lucide power icon instead of square for stop/turn-off buttons
- Add accent-tinted hover backgrounds for start (green) and stop (amber)

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

server/src/wled_controller/static/css/dashboard.css

@@ -202,6 +202,36 @@
     gap: 4px;
 }
 
+.dashboard-action-btn {
+    background: none;
+    border: none;
+    width: 28px;
+    height: 28px;
+    padding: 0;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    cursor: pointer;
+    border-radius: 4px;
+    transition: color 0.2s, background 0.2s;
+}
+
+.dashboard-action-btn.start {
+    color: var(--primary-color);
+}
+
+.dashboard-action-btn.start:hover {
+    background: color-mix(in srgb, var(--primary-color) 12%, transparent);
+}
+
+.dashboard-action-btn.stop {
+    color: var(--warning-color, #ffc107);
+}
+
+.dashboard-action-btn.stop:hover {
+    background: rgba(255, 193, 7, 0.12);
+}
+
 .dashboard-autostart-btn {
     background: none;
     border: none;