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/view.go

@@ -139,7 +139,8 @@ func (m Model) renderFooter() string {
 	case m.editMode == ModeInsert:
 		hints = []string{hint("enter", "send"), hint("esc", "normal"), hint("↑↓", "history")}
 	case m.displayState() == StateApproval:
-		hints = []string{hint("a", "approve"), hint("A", "auto"), hint("s", "steer"), hint("r", "reject"), hint("x", "diff"), hint("esc", "later")}
+		// Approval actions live in the band itself; the footer offers navigation only.
+		hints = []string{hint("l", "back"), hint("e", "events"), hint("q", "quit")}
 	case m.displayState() == StateIdle:
 		hints = []string{hint("i", "name"), hint("/", "filter"), hint("enter", "open"), hint("jk", "move"), hint("w", "workflows"), hint("p", "cmds"), hint("q", "quit")}
 	default: // StateInSession