refactor(tui-go): move session dates to the R overlay; drop dead idle panels

The launcher removed the idle session list, so the per-row date moves to the R
resume overlay — now the sole session list. sessionListRow shows an absolute
local date (absDateISO → shortDateTime) alongside the relative age:
"Jun 22 14:30 · 5m ago", with the year shown for older-than-this-year rows.

Delete the now-unused idle-panel renderers (sessionRows / welcomeRows /
workflowRows) and renderMainNarrow's dead idle branch (renderLauncher owns idle,
narrow included). Add a "resume" preview kind to screenshot the R overlay.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -228,10 +228,25 @@ func (m Model) sessionListRow(i int) string {
 	}
 	stageCell := mbg(t, padRaw(stage, 22), t.P.Accent2)
 
-	age := relativeAge(s.LastActivityAt)
-	ageCell := mbg(t, age, t.P.Faint)
+	dateCell := mbg(t, padRaw(absDateISO(s.LastActivityAt), 12), t.P.Dim)
+	ageCell := mbg(t, "· "+relativeAge(s.LastActivityAt), t.P.Faint)
 
-	return marker + idCell + " " + statusCell + " " + stageCell + " " + ageCell
+	return marker + idCell + " " + statusCell + " " + stageCell + " " + dateCell + " " + ageCell
+}
+
+// absDateISO renders an ISO-8601 instant (the server's LastActivityAt) as a compact local
+// date via shortDateTime, or "—" when absent/unparseable.
+func absDateISO(iso string) string {
+	if iso == "" {
+		return "—"
+	}
+	ts, err := time.Parse(time.RFC3339Nano, iso)
+	if err != nil {
+		if ts, err = time.Parse(time.RFC3339, iso); err != nil {
+			return "—"
+		}
+	}
+	return shortDateTime(ts.UnixMilli())
 }
 
 // shortSessionID trims a long session id to a recognizable prefix for the list.