Visualizer: add a per-item visibility toggle to the workcell tree

[BioCam]

The visualizer's workcell tree lists every resource in the workcell - the liquid handler, carriers, plates, tip racks, and lids - but offered no way to hide an individual item from the canvas. On a dense deck, resources overlap and a carrier or lid occludes whatever sits beneath it, with no way to declutter the view or look underneath short of editing the layout.

• Add an eye toggle to every row of the workcell tree (including the root); clicking it hides or shows that resource on the canvas.
• Hiding flips the resource's Konva group visibility instead of destroying and redrawing it, so the item keeps its exact location and stacking order when shown again.
• Hiding a container hides everything on it: the canvas scene graph mirrors the resource tree, so a hidden group clips all of its descendants - hide a carrier and its racks and plates go with it.
• The hidden set is keyed by resource name and re-applied on every redraw, so an item stays hidden across the volume changes, tip pickups, and re-assignments that rebuild its canvas group.
• A row whose parent is hidden is shown disabled and greyed with an explanatory tooltip, because its own toggle cannot take effect while an ancestor hides it; alt-clicking such a row reveals it by clearing the hidden flag on it and every hidden ancestor.

Behavior: nothing changes until an eye is clicked - the default view, canvas rendering, and tree counts are untouched. The toggle is view-only; it never alters the deck layout or any resource state, only what the canvas draws, so it is a safe way to declutter a busy deck and inspect occluded or nested resources. Frontend only (lib.js and main.css); no Python or API change.

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