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/demo.go

@@ -22,6 +22,16 @@ func PreviewFrame(kind string, w, h int) string {
 		m.bgUpdates = 3
 		m.selectedID = "04a546aa"
 
+	case "resume":
+		m.connected = true
+		m.currentModel = "llama-cpp:default"
+		m.overlay = OverlaySessions
+		m.sessionList = []SessionSummary{
+			{SessionID: "04a546aa8b2c", Status: "ACTIVE", WorkflowID: "healthcheck", StageCount: 3, LastActivityAt: "2026-06-22T14:30:05Z"},
+			{SessionID: "0d7097bb1f3e", Status: "COMPLETED", WorkflowID: "healthcheck", StageCount: 5, LastActivityAt: "2026-06-21T09:15:00Z"},
+			{SessionID: "1dae17cc77aa", Status: "FAILED", WorkflowID: "chat", LastActivityAt: "2025-12-30T18:02:00Z"},
+		}
+
 	case "workflows":
 		m.connected = true
 		m.currentModel = "llama-cpp:default"