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/model.go

@@ -210,25 +210,21 @@ func NewModel(client *ws.Client) Model {
 	}
 }
 
-// displayState derives the active screen, matching the Kotlin extension.
+// displayState derives the active screen. A session must be *entered*
+// (sessionEntered) before its in-session or approval surfaces show — otherwise
+// merely moving the list cursor onto a session with a pending gate would yank
+// you into the approval, and `l` back-to-list couldn't escape it.
 func (m Model) displayState() DisplayState {
-	if m.selectedID == "" {
+	if m.selectedID == "" || !m.sessionEntered {
 		return StateIdle
 	}
-	hasApproval := false
+	if m.approvalDismissed {
+		return StateInSession
+	}
 	if s := m.session(m.selectedID); s != nil && s.Pending != nil {
-		hasApproval = true
-	}
-	switch {
-	case m.approvalDismissed:
-		return StateInSession
-	case hasApproval:
 		return StateApproval
-	case m.sessionEntered:
-		return StateInSession
-	default:
-		return StateIdle
 	}
+	return StateInSession
 }
 
 func (m Model) session(id string) *Session {