Add the "HA Dashboard" tab with HA's web UI (DSH-1)

A second dashboard surface shows a WebEngineView at homeassistant.url, or
a hint when none is set. HaWebUi (QML singleton owned by Dashboard) holds
the URL and a persistent WebEngine profile under
~/.local/share/frameassistant/webengine so the HA login survives restarts.
QtWebEngineQuick::initialize() runs in main() before the app object (D39).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ha/HaWebUi.cpp

@@ -0,0 +1,49 @@
+#include "ha/HaWebUi.h"
+
+#include <QQmlEngine>
+#include <QQuickWebEngineProfile>
+#include <QStandardPaths>
+
+namespace {
+
+HaWebUi* g_instance = nullptr;
+
+}  // namespace
+
+HaWebUi::HaWebUi(QObject* parent) : QObject(parent) {
+  Q_ASSERT(!g_instance);
+  g_instance = this;
+}
+
+HaWebUi::~HaWebUi() { g_instance = nullptr; }
+
+QQuickWebEngineProfile* HaWebUi::profile() {
+  if (!m_profile) {
+    m_profile = new QQuickWebEngineProfile(this);
+    m_profile->setStorageName(QStringLiteral("frameassistant"));
+    m_profile->setOffTheRecord(false);
+    m_profile->setPersistentStoragePath(storagePath());
+    m_profile->setCachePath(cachePath());
+    m_profile->setPersistentCookiesPolicy(
+        QQuickWebEngineProfile::AllowPersistentCookies);
+  }
+  return m_profile;
+}
+
+QString HaWebUi::storagePath() {
+  return QStandardPaths::writableLocation(QStandardPaths::GenericDataLocation) +
+         QStringLiteral("/frameassistant/webengine");
+}
+
+QString HaWebUi::cachePath() {
+  return QStandardPaths::writableLocation(
+             QStandardPaths::GenericCacheLocation) +
+         QStringLiteral("/frameassistant/webengine");
+}
+
+HaWebUi* HaWebUi::create(QQmlEngine*, QJSEngine*) {
+  Q_ASSERT(g_instance);
+  // Owned by the app, not by the QML engine.
+  QJSEngine::setObjectOwnership(g_instance, QJSEngine::CppOwnership);
+  return g_instance;
+}

src/ha/HaWebUi.h

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+#pragma once
+
+#include <QtQml/qqmlregistration.h>
+
+#include <QObject>
+#include <QQuickWebEngineProfile>
+#include <QUrl>
+
+class QJSEngine;
+class QQmlEngine;
+
+// HA's web UI in the "HA Dashboard" tab (DSH-1): the URL from the settings
+// and a persistent WebEngine profile, so the HA login survives restarts
+// (docs/DESIGN.md, D39). A QML singleton owned by the app (Dashboard).
+class HaWebUi : public QObject {
+  Q_OBJECT
+  QML_ELEMENT
+  QML_SINGLETON
+  // Empty: no URL configured (the tab shows a hint).
+  Q_PROPERTY(QUrl url READ url CONSTANT)
+  Q_PROPERTY(QQuickWebEngineProfile* profile READ profile CONSTANT)
+
+ public:
+  // Not default-constructible, so QML uses create() instead of making its
+  // own instance.
+  explicit HaWebUi(QObject* parent);
+  ~HaWebUi() override;
+
+  // Call before the QML is loaded.
+  void setUrl(const QUrl& url) { m_url = url; }
+
+  QUrl url() const { return m_url; }
+  // Created on first use: only a tab that shows the web UI needs it.
+  QQuickWebEngineProfile* profile();
+
+  // Where the profile keeps its data (cookies, local storage) and cache.
+  static QString storagePath();
+  static QString cachePath();
+
+  // QML singleton factory: the instance the app created.
+  static HaWebUi* create(QQmlEngine*, QJSEngine*);
+
+ private:
+  QUrl m_url;
+  QQuickWebEngineProfile* m_profile = nullptr;
+};