Stop node components re-rendering on every frame of a drag

[Vein05]

Issue

React Flow passes each node component its position as props, so the node
being dragged re-renders its whole Mantine subtree on every pointer move.
None of the node components read the position -- they take only
{ data, id }, plus type for CodeEvaluatorNode, which serves two node types.

Not the same as #458: that was a node subscribing to the whole nodes array,
so nodes standing still re-rendered. This is the dragged node re-rendering
itself, and it costs the same at 3 nodes as at 48.

Fix

A helper, src/memoNode.tsx, wraps a node component in React.memo comparing
only the props these components read:

(a, b) => a.id === b.id && a.data === b.data && a.type === b.type

and each of the 21 node components exports through it:

export default memoNode(PromptNode);

Comparing data by identity works because setDataPropsForNode replaces it
with a copy (store.tsx:1100). The helper's docstring notes the constraint: a
node component that starts reading selected or dragging has to compare
them here too.

Numbers

Two production builds served side by side and profiled in one browser
session (Chrome CPU profile over CDP), dragging a node for ~2.5s, two runs
each, M-series Mac. Share of wall clock off-idle, and total off-idle CPU:

                   before            after
prompt node        47%  (1374 ms)    23%  (546 ms)
table node         46%  (1079 ms)    22%  (491 ms)
inspect node       26%  ( 561 ms)    21%  (461 ms)

The prompt node went from ~23 ms of CPU per frame to ~9 ms, so a drag that
could not hold 60fps now fits in the frame budget. Each node type pays this
cost when it is the one being dragged, which is why the table node gains as
much as the prompt node.

Checked

prettier -c, eslint, tsc --noEmit and craco test (68 suites, 1234 tests) pass.