Ports the vanilla spawn form/modal to src/components/spawn/:
SpawnModal, SpawnForm, DirAutocomplete, BranchPicker, and a
useFastForward hook for the one-keystroke spawn. Pure logic modules
(spawnForm.ts's buildSpawnRequest, fastForward.ts, spawnSimilar.ts,
projectList.ts, toolIcon.ts) are reused as-is rather than re-derived.
Exports supportsStructuredView from spawnForm.ts so the React form can
share the structured-view-eligibility check with the vanilla one.
