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/diff.go

@@ -78,6 +78,48 @@ func parseUnifiedDiff(diff string) []diffRow {
 	return rows
 }
 
+// isUnifiedDiff reports whether s looks like a unified diff (file/hunk headers)
+// rather than arbitrary preview text such as a shell argv JSON. Non-diff previews
+// must not go through the two-column renderer (it would show identical columns).
+func isUnifiedDiff(s string) bool {
+	return strings.Contains(s, "@@ -") || strings.HasPrefix(s, "--- ") || strings.Contains(s, "\n--- ")
+}
+
+// previewPlainLines splits non-diff preview text into display lines.
+func previewPlainLines(s string) []string {
+	return strings.Split(strings.TrimRight(s, "\n"), "\n")
+}
+
+// previewRowCount is the number of content rows a preview occupies — diff rows
+// for a real diff, plain lines otherwise. Drives band sizing and scroll bounds.
+func previewRowCount(s string) int {
+	if isUnifiedDiff(s) {
+		return len(parseUnifiedDiff(s))
+	}
+	return len(previewPlainLines(s))
+}
+
+// renderPreviewLines renders non-diff preview text as dim plain lines (single
+// column), up to maxRows starting at off, each truncated to width.
+func (m Model) renderPreviewLines(content string, width, maxRows, off int) []string {
+	lines := previewPlainLines(content)
+	if off < 0 {
+		off = 0
+	}
+	if off > len(lines) {
+		off = len(lines)
+	}
+	end := off + maxRows
+	if end > len(lines) {
+		end = len(lines)
+	}
+	out := make([]string, 0, end-off)
+	for _, ln := range lines[off:end] {
+		out = append(out, m.theme.span(truncate(ln, width), m.theme.P.Dim))
+	}
+	return out
+}
+
 // hunkStarts reads the starting old/new line numbers from an @@ -a,b +c,d @@ marker.
 func hunkStarts(h string) (int, int) {
 	old, newer := 1, 1