fix(tui): wrap long artifact lines in viewer instead of truncating

Artifact content wider than the modal was silently cut off. Lines are now
hard-wrapped at rune boundaries to the modal content width, and scroll
clamping uses the wrapped line count so the last page stays reachable.

apps/tui-go/internal/app/update.go

@@ -401,7 +401,8 @@ func (m Model) handleOverlayKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 				m.artifactScroll--
 			}
 		case k.Type == tea.KeyPgDown:
-			if m.artifactScroll < m.artifactContentMaxScroll() {
+			contentW := m.modalWidth() - 6
+			if m.artifactScroll < m.artifactContentMaxScroll(contentW) {
 				m.artifactScroll++
 			}
 		case runeIs(k, "v"):