fix(tui): clamp diff scroll offset to last full page

The diff overlay incremented diffScrollOffset on every down-key with no
upper bound; the render clamped only the displayed offset, so the stored
value grew unbounded and the up-key appeared dead for N presses. Clamp
the increment against a new diffMaxScroll() (len(lines) - body height),
shared with the render path via diffBodyHeight().

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

@@ -332,7 +332,9 @@ func (m Model) handleOverlayKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 				m.diffScrollOffset--
 			}
 		case k.Type == tea.KeyDown || runeIs(k, "j"):
-			m.diffScrollOffset++
+			if m.diffScrollOffset < m.diffMaxScroll() {
+				m.diffScrollOffset++
+			}
 		case k.Type == tea.KeyCtrlX:
 			m.overlay = OverlayNone
 		}