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/components.css

@@ -398,6 +398,7 @@ input:-webkit-autofill:focus {
 .header-btn:focus-visible,
 .tutorial-trigger-btn:focus-visible,
 .tutorial-close-btn:focus-visible,
+.dashboard-action-btn:focus-visible,
 .btn-expand-collapse:focus-visible,
 .btn-filter-action:focus-visible,
 .settings-toggle:focus-visible {