Give empty overlays zero list rows and an explicit empty state

[TheGreatAxios]

Summary

• Empty overlays reserve zero list rows and paint an explicit empty state inside the body chrome
• Short-terminal layout holds no phantom choice row for overlays with nothing to choose

Verification

• bun run typecheck, bun run build, and bun run test pass
• New geometry, paint, and short-terminal coverage in src/tui/overlay-empty-state.test.ts passes

Fixes CL-6720

[linear-code]

CL-6720

[TheGreatAxios]

Audit: PASS with one requested test (empty-accept no-op)

Verified: zero list rows for empty overlays, explicit (no choices) empty state inside the body chrome (distinct from the (no matches) filter sentinel, which stays a real choice row), no phantom choice row on a short terminal, and the setOverlayBody / setOwnedOverlayItems refresh paths collapsing to zero rows. The Math.max(1, …) → Math.max(0, …) conversions and the hasItems gate in fitOverlayListToHost are the complete set — no other max(1, …) floor remains on this path.

Finding (test only, no product change): openListOverlay still passes activeIndex: 0 with empty labels, and acceptOverlaySelection already guards it (overlayItems.length === 0 is a no-op — it stays open — unless a live gate with no answer field fail-closes via onAccept with no id). That guard has no regression test. I will add one to this branch: accept on an empty overlay fires no accept callback and leaves the overlay open with zero rows.