fix(tui): approval nav lockups, shell non-diff preview, hint dedup

Three QA-found issues in the approval gate:

- displayState gated the in-session/approval surfaces on hasApproval
  before sessionEntered, so moving the list cursor onto a session with a
  pending gate auto-opened it, and `l` back-to-list couldn't escape
  (the gate re-popped). Require sessionEntered first.
- The band ran every preview through the two-column diff renderer, so a
  shell tool's argv JSON rendered as an identical-column "diff". Detect
  real unified diffs (isUnifiedDiff); render other previews as plain
  text, and drop the ^x fullscreen hint when there's nothing to expand.
- The footer duplicated the band's approve/reject/steer/diff keys. It now
  shows navigation (l back / e events / q quit) instead.

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

@@ -58,7 +58,7 @@ func mbg(t Theme, s string, fg lipgloss.Color) string {
 func (m Model) approvalBandHeight() int {
 	rows, rat := 0, 0
 	if s := m.session(m.selectedID); s != nil && s.Pending != nil {
-		rows = len(parseUnifiedDiff(s.Pending.Preview))
+		rows = previewRowCount(s.Pending.Preview)
 		if n := len(s.Pending.Rationale); n > 0 {
 			rat = n + 1 // rationale lines + trailing blank
 		}
@@ -114,8 +114,6 @@ func (m Model) renderApprovalBand(h int) string {
 	if diffH < 1 {
 		diffH = 1
 	}
-	rows := parseUnifiedDiff(a.Preview)
-
 	body := make([]string, 0, innerH)
 	body = append(body, m.justify(leftHdr, rightHdr, textW, t.P.Bg), "")
 	for _, r := range a.Rationale {
@@ -125,8 +123,14 @@ func (m Model) renderApprovalBand(h int) string {
 	if len(a.Rationale) > 0 {
 		body = append(body, "")
 	}
+	var content []string
+	if isUnifiedDiff(a.Preview) {
+		content = m.renderSplitDiff(parseUnifiedDiff(a.Preview), textW, diffH, 0)
+	} else {
+		content = m.renderPreviewLines(a.Preview, textW, diffH, 0)
+	}
 	diffStart := len(body)
-	body = append(body, m.renderSplitDiff(rows, textW, diffH, 0)...)
+	body = append(body, content...)
 	for len(body) < diffStart+diffH {
 		body = append(body, "")
 	}
@@ -150,8 +154,11 @@ func (m Model) approvalActions() string {
 		return lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.Bg).Bold(true).Render(k) +
 			t.span(" "+l, t.P.Faint)
 	}
-	parts := []string{hint("a", "approve"), hint("A", "auto"), hint("s", "steer"),
-		hint("r", "reject"), hint("^x", "fullscreen"), hint("esc", "later")}
+	parts := []string{hint("a", "approve"), hint("A", "auto"), hint("s", "steer"), hint("r", "reject")}
+	if s := m.session(m.selectedID); s != nil && s.Pending != nil && isUnifiedDiff(s.Pending.Preview) {
+		parts = append(parts, hint("^x", "fullscreen"))
+	}
+	parts = append(parts, hint("esc", "later"))
 	return strings.Join(parts, t.span("   ", t.P.Faint))
 }
 
@@ -167,8 +174,7 @@ func (m Model) diffBodyHeight() int {
 // diffMaxScroll is the largest scroll offset that still fills the body with
 // diff rows, keeping the last page anchored to the bottom of the modal.
 func (m Model) diffMaxScroll() int {
-	rows := parseUnifiedDiff(m.currentDiff())
-	max := len(rows) - m.diffBodyHeight()
+	max := previewRowCount(m.currentDiff()) - m.diffBodyHeight()
 	if max < 0 {
 		max = 0
 	}
@@ -180,7 +186,7 @@ func (m Model) diffModal() string {
 	w := m.modalWidth()
 	bodyH := m.diffBodyHeight()
 	diff := m.currentDiff()
-	rows := parseUnifiedDiff(diff)
+	isDiff := isUnifiedDiff(diff)
 
 	off := m.diffScrollOffset
 	if off > m.diffMaxScroll() {
@@ -191,12 +197,22 @@ func (m Model) diffModal() string {
 	}
 
 	var b strings.Builder
-	b.WriteString(m.titleLine("diff"))
+	title := "preview"
+	if isDiff {
+		title = "diff"
+	}
+	b.WriteString(m.titleLine(title))
 	if tgt := diffTarget(diff); tgt != "" {
 		b.WriteString(mbg(t, "  "+tgt, t.P.Dim))
 	}
-	b.WriteString(mbg(t, "   ("+itoa(len(rows))+" rows)", t.P.Faint) + "\n\n")
-	for _, ln := range m.renderSplitDiff(rows, w-6, bodyH, off) {
+	b.WriteString(mbg(t, "   ("+itoa(previewRowCount(diff))+" rows)", t.P.Faint) + "\n\n")
+	var lines []string
+	if isDiff {
+		lines = m.renderSplitDiff(parseUnifiedDiff(diff), w-6, bodyH, off)
+	} else {
+		lines = m.renderPreviewLines(diff, w-6, bodyH, off)
+	}
+	for _, ln := range lines {
 		b.WriteString(ln + "\n")
 	}
 	b.WriteString("\n" + modalHints(t, [][2]string{{"↑↓", "scroll"}, {"^x/esc", "close"}}))