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/overlays.go

@@ -94,20 +94,36 @@ func (m Model) renderApproval(base string) string {
 	return m.center(modal)
 }
 
-func (m Model) diffModal() string {
-	t := m.theme
-	w := m.modalWidth()
+// diffBodyHeight is the number of diff lines visible in the diff modal body.
+func (m Model) diffBodyHeight() int {
 	h := m.height * 70 / 100
 	if h < 8 {
 		h = 8
 	}
-	bodyH := h - 6
+	return h - 6
+}
+
+// diffMaxScroll is the largest scroll offset that still fills the body with
+// diff lines, keeping the last page anchored to the bottom of the modal.
+func (m Model) diffMaxScroll() int {
+	lines := strings.Split(strings.TrimRight(m.currentDiff(), "\n"), "\n")
+	max := len(lines) - m.diffBodyHeight()
+	if max < 0 {
+		max = 0
+	}
+	return max
+}
+
+func (m Model) diffModal() string {
+	t := m.theme
+	w := m.modalWidth()
+	bodyH := m.diffBodyHeight()
 	diff := m.currentDiff()
 	lines := strings.Split(strings.TrimRight(diff, "\n"), "\n")
 
 	off := m.diffScrollOffset
-	if off > len(lines)-1 {
-		off = len(lines) - 1
+	if off > m.diffMaxScroll() {
+		off = m.diffMaxScroll()
 	}
 	if off < 0 {
 		off = 0