v0.9.1: the window opens on the session the engine already has open

A workspace is never empty on the second run: cst_agent_open() resumes the
last session of the store, so the engine holds a transcript before the window
exists. The window only filled its view on a session *switch*, and a start is
not a switch -- the pane came up empty next to a sidebar pointing at the very
session it belonged to (on the phone: "hello, 6 messages" highlighted, no
transcript under it).

- Main.qml: Component.onCompleted reloads the session list and replays the
  open session, the same call the sessionChanged handler makes.
- AgentCore::switchSession(): a click on the row of the session that is
  already open emits sessionChanged instead of returning quietly, so the row
  redraws its transcript rather than being a dead spot.
- --uitest: the window is now created over a seeded session (a fixture is
  written and switched to before the QML loads, which is what a restart looks
  like) and the check "the open session is on screen at startup" pins it --
  it failed with 0 rows before the fix. Two qWaits in the settings-dialog
  section, whose clicks were racing the dialog's layout: the extra startup
  work made that visible (5 failures that had nothing to do with sessions).

Verified: --uitest 79/79 (the new check among them), cst-app-selftest OK, and
on the phone -- the APK rebuilt and installed, a restart of the app draws the
whole stored session (messages, tool cards, thinking blocks, copy icons).

Version 0.9.1.

mode = coding

app/src/core/AgentCore.cpp

@@ -551,8 +551,13 @@ bool AgentCore::newSession() {
 bool AgentCore::switchSession(const QString &id) {
   if (!m_agent || m_running || id.isEmpty())
     return false;
-  if (id == sessionId())
+  if (id == sessionId()) {
+    /* A click on the row of the session that is already open: nothing to
+     * switch, but the window still wants the transcript (the row is not a
+     * dead spot), and sessionChanged is what draws it. */
+    emit sessionChanged();
     return true;
+  }
   if (cst_agent_switch_session(m_agent, id.toUtf8().constData()) != 0)
     return false;
   afterSessionChange();