Hide nodes

[milanofthe]

Closes #154.

Summary

• Nodes can be hidden via right-click → Hide (single or multi-select), or the eye-off button in the Properties dialog. Hidden nodes are stored as _hidden: true on the node params, so the state persists with the .pvm file and participates in undo/redo.
• FlowCanvas filters hidden nodes out of the visible set; any edge whose source or target is hidden is also filtered, automatically reappearing on unhide.
• Simulation is not affected — hidden nodes are still part of the graph store and run normally (Scopes keep recording etc.). The feature is a pure visibility filter.
• Toolbar gains an eye-off button with a counter badge, shown only when something is hidden. Hover reveals a menu listing hidden nodes (name + type); click an entry to unhide it, or "Show all" to unhide everything in the current context.
• Interface blocks are excluded from the Hide affordance — hiding them would break the subsystem contract.