fix: block neighbor expansion while the limit input is empty

[vishwakt]

Fixes #2068

Clearing the limit field stored NaN, because parseInt("") returns NaN and ?? only catches null or undefined. The expand request treats a falsy limit as no limit, so the request went out unlimited while the toggle still showed a limit was applied.

The limit state now stores null for an empty or invalid input, the field renders empty instead of value={NaN}, and the Expand button is disabled until a valid limit is entered or the toggle is turned off.

Verified against a local TinkerPop Gremlin Server through the proxy server, with a node seeded with 303 neighbors. With the field cleared, the outgoing Gremlin query on main had no .range() clause. With a valid limit of 10 on this branch, the query contains .range(0, 10) and exactly 10 neighbors arrive. Added a component test covering the cleared and repopulated input.

Before, the setup: limit toggle on, field cleared, Expand still enabled:

Before, the result: one click pulls all 301 unfetched neighbors onto the canvas while the toggle says a limit is applied:

After: same state, Expand is disabled until a valid limit is entered or the toggle is turned off:

After, with a valid limit: entering 10 and expanding adds exactly 10 neighbors, so the normal flow is unchanged: