Make panel header bar sticky

[Copilot]

The panel header bar (title + hamburger menu button) scrolled out of view when the container list was long enough to require scrolling.

Change

Added sticky positioning to the .toolbar CSS rule in frontend/ssh-docker-panel.js:

.toolbar {
  /* existing properties */
  position: sticky;
  top: 0;
  z-index: 10;
}

z-index: 10 ensures the bar renders above the scrolling content beneath it.

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