Fix nodes and connections blinking after every scene update

[dovvnloading]

Problem

While moving nodes around the canvas, their connection lines were visibly failing to keep up: they would drop out for a moment and then reappear, and the node card itself could briefly vanish. The effect was most noticeable on whichever node the user had just selected or dragged, and it also showed up during streamed responses and shortly after panning or zooming.

The cause is in how the canvas rebuilds its node list whenever the backend sends a scene update. React Flow, the library that renders the canvas, records each node's on-screen size on the node object itself once it has measured it. Our rebuild produced fresh node objects that did not carry that recorded size forward. When React Flow received a node without its size, it treated the node as never measured: it hid the node until it could measure it again, and it removed every connection attached to it, because a connection cannot be drawn to an unmeasured node. The re-measurement takes at least one frame, so each scene update produced a visible blink. Scene updates happen constantly in normal use - after every drag is released, after every pan or zoom is reported to the backend, and on every streamed chunk of a response - so the blinking read as the canvas constantly stuttering.

An existing step in the rebuild made this worse. The function that preserves the user's selection across updates created a fresh copy of every selected node on every update, which meant the selected node lost its recorded size even when nothing about it had actually changed.

Change

The selection-preserving step (withPreservedSelection, now renamed withPreservedFlowState) carries the recorded size across the rebuild along with the selection flag. With the size present, React Flow keeps the node's existing geometry: the node stays visible and its connections stay drawn while any re-measurement happens in the background. Nodes that come out of the rebuild as the exact same object as before pass through untouched, keeping the existing fast path for unchanged nodes.

Each step of the failure was confirmed by reading the installed React Flow source rather than assumed: when a node object is replaced, the library rebuilds its internal record for that node; it keeps the connection-point geometry only if the incoming object carries a recorded size; a connection to an unmeasured node is not drawn at all; and a node without dimensions is rendered invisible.

Test plan

• Full frontend check passes end to end: schema check, type check, lint (0 errors), 1927/1927 tests, production build, bundle-size check
• Five new unit tests cover the size carry-over: it is applied to rebuilt nodes, combined correctly with selection, never invented when no size was recorded, never overrides a size the rebuilt node already has, and never breaks the untouched fast path for unchanged nodes
• All existing selection-preservation tests pass unchanged against the renamed function