Swap keyword-search attribute picker to the virtualized Combobox

## Parent

- Part of #2086

## What to build

Swap the keyword-search **attribute** select from the raw Radix `Select` to the virtualized searchable Combobox. On a graph whose selected vertex type has a large searchable-attribute set, the attribute picker opens instantly, filters as the user types, and selects, feeling instant because the attribute derivation is now memoized.

## Acceptance criteria

- [ ] Keyword-search attribute select uses the Combobox, preserving its existing `selectedAttributeAtom`-controlled wiring
- [ ] Instant open + filter + select on a large attribute set
- [ ] `pnpm checks` and `pnpm test` pass

## Blocked by

- #2087 (Combobox component)
- #2088 (useSearchableAttributes memoization)

Start at the keyword-search attribute select and trace its existing selectedAttributeAtom-controlled wiring. Read the Combobox work in #2087 and the useSearchableAttributes memoization in #2088, then run pnpm checks and pnpm test. Done means the picker uses the Combobox, opens and filters instantly on large attribute sets, selects correctly, and both commands pass.

Written by the indexing model from the issue text.