feat(tui-go): session list / resume view (E)

R opens a navigable overlay of recent sessions (GET /sessions: short id, status,
workflow/stage, relative age); enter resumes via POST /sessions/{id}/resume, which
replays onto the existing global /stream (no WS re-dial). Fetch/resume errors surface
in the overlay, never panic. stdlib-only (ws.Client.HTTPBase derives the base URL).

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

apps/tui-go/internal/ws/client.go

@@ -28,23 +28,36 @@ type Status struct {
 // Client is a reconnecting WebSocket client. Construct with New, then Run in a
 // goroutine. Incoming and Conn are drained by the UI.
 type Client struct {
-	url  string
-	send chan []byte
-	in   chan protocol.ServerMessage
-	conn chan Status
+	url      string
+	httpBase string
+	send     chan []byte
+	in       chan protocol.ServerMessage
+	conn     chan Status
 }
 
 // New builds a client targeting ws://host:port/stream.
 func New(host string, port int) *Client {
-	u := url.URL{Scheme: "ws", Host: hostPort(host, port), Path: "/stream"}
+	hp := hostPort(host, port)
+	u := url.URL{Scheme: "ws", Host: hp, Path: "/stream"}
 	return &Client{
-		url:  u.String(),
-		send: make(chan []byte, 64),
-		in:   make(chan protocol.ServerMessage, 256),
-		conn: make(chan Status, 16),
+		url:      u.String(),
+		httpBase: (&url.URL{Scheme: "http", Host: hp}).String(),
+		send:     make(chan []byte, 64),
+		in:       make(chan protocol.ServerMessage, 256),
+		conn:     make(chan Status, 16),
 	}
 }
 
+// HTTPBase is the http://host:port origin the WS client connects to, for REST
+// calls (e.g. GET /sessions) that share the server's host/port. Empty for a nil
+// client (the test harness constructs Models with a nil client).
+func (c *Client) HTTPBase() string {
+	if c == nil {
+		return ""
+	}
+	return c.httpBase
+}
+
 func hostPort(host string, port int) string {
 	h := host
 	if h == "" {