Custom port labels

[milanofthe]

Closes #289.

Summary

• New updateNodePortName(nodeId, direction, index, name) store action writes the new label directly to node.{inputs|outputs}[index].name. Port names are already part of NodeInstance and persist in the .pvm file — no new params introduced.
• For Interface blocks the write is redirected to the parent Subsystem (with the direction flipped, since Interface input ↔ Subsystem output). The Interface itself derives its port names from the parent at read time, so custom labels propagate both ways automatically.
• Properties dialog gets a third view (alongside Properties / Python Code) — toggled via a tag icon in the header. Title swaps to "Port Labels", body shows one input per port with the default name as placeholder. Code view and Port-Labels view are mutually exclusive.
• Hidden for blocks whose port names are already driven by a regular param (Scope.labels, Spectrum.labels, Adder.operations, Divider.operations) — there the param is the single source of truth.
• Separator between input and output entries when both exist; spans full dialog width like other section separators.